diff --git a/core/server/api/canary/posts.js b/core/server/api/canary/posts.js
index c87297c4ec..f571be5ec3 100644
--- a/core/server/api/canary/posts.js
+++ b/core/server/api/canary/posts.js
@@ -2,7 +2,7 @@ const models = require('../../models');
 const tpl = require('@tryghost/tpl');
 const errors = require('@tryghost/errors');
 const getPostServiceInstance = require('../../services/posts/posts-service');
-const allowedIncludes = ['tags', 'authors', 'authors.roles', 'email', 'tiers'];
+const allowedIncludes = ['tags', 'authors', 'authors.roles', 'email', 'tiers', 'newsletter'];
 const unsafeAttrs = ['status', 'authors', 'visibility'];
 
 const messages = {
diff --git a/core/server/api/canary/utils/serializers/input/posts.js b/core/server/api/canary/utils/serializers/input/posts.js
index e8c7f15e9b..c5c1b427a8 100644
--- a/core/server/api/canary/utils/serializers/input/posts.js
+++ b/core/server/api/canary/utils/serializers/input/posts.js
@@ -38,7 +38,7 @@ function defaultRelations(frame) {
         return false;
     }
 
-    frame.options.withRelated = ['tags', 'authors', 'authors.roles', 'email', 'tiers'];
+    frame.options.withRelated = ['tags', 'authors', 'authors.roles', 'email', 'tiers', 'newsletter'];
 }
 
 function setDefaultOrder(frame) {
diff --git a/core/server/api/canary/utils/serializers/output/mappers/posts.js b/core/server/api/canary/utils/serializers/output/mappers/posts.js
index 3a9c708ff7..7a96f1d2ef 100644
--- a/core/server/api/canary/utils/serializers/output/mappers/posts.js
+++ b/core/server/api/canary/utils/serializers/output/mappers/posts.js
@@ -96,6 +96,10 @@ module.exports = async (model, frame, options = {}) => {
             if (relation === 'email' && _.isEmpty(jsonModel.email)) {
                 jsonModel.email = null;
             }
+
+            if (relation === 'newsletter' && _.isEmpty(jsonModel.newsletter)) {
+                jsonModel.newsletter = null;
+            }
         });
     }
 
diff --git a/core/server/api/canary/utils/serializers/output/utils/clean.js b/core/server/api/canary/utils/serializers/output/utils/clean.js
index b8c8ac6f78..05a7b5d21f 100644
--- a/core/server/api/canary/utils/serializers/output/utils/clean.js
+++ b/core/server/api/canary/utils/serializers/output/utils/clean.js
@@ -77,6 +77,7 @@ const post = (attrs, frame) => {
         // delete attrs.page;
         delete attrs.status;
         delete attrs.email_only;
+        delete attrs.newsletter;
 
         // We are standardising on returning null from the Content API for any empty values
         if (attrs.twitter_title === '') {
@@ -124,6 +125,7 @@ const post = (attrs, frame) => {
     delete attrs.locale;
     delete attrs.author;
     delete attrs.type;
+    delete attrs.newsletter_id;
 
     return attrs;
 };
diff --git a/package.json b/package.json
index f6f3295e2b..54eca544da 100644
--- a/package.json
+++ b/package.json
@@ -57,11 +57,11 @@
   "dependencies": {
     "@sentry/node": "6.19.7",
     "@tryghost/adapter-manager": "0.2.30",
-    "@tryghost/admin-api-schema": "2.14.1",
+    "@tryghost/admin-api-schema": "2.15.0",
     "@tryghost/api-version-compatibility-service": "0.2.0",
     "@tryghost/bookshelf-plugins": "0.4.1",
     "@tryghost/bootstrap-socket": "0.2.19",
-    "@tryghost/color-utils": "0.1.14",
+    "@tryghost/color-utils": "0.1.15",
     "@tryghost/config-url-helpers": "0.1.7",
     "@tryghost/constants": "1.0.4",
     "@tryghost/custom-theme-settings-service": "0.3.2",
@@ -73,7 +73,7 @@
     "@tryghost/email-content-generator": "0.1.1",
     "@tryghost/errors": "1.2.12",
     "@tryghost/express-dynamic-redirects": "0.2.11",
-    "@tryghost/helpers": "1.1.65",
+    "@tryghost/helpers": "1.1.66",
     "@tryghost/image-transform": "1.0.31",
     "@tryghost/job-manager": "0.8.23",
     "@tryghost/kg-card-factory": "3.1.3",
diff --git a/test/e2e-api/admin/posts.test.js b/test/e2e-api/admin/posts.test.js
index 3cef5d473f..4ad0f2b46e 100644
--- a/test/e2e-api/admin/posts.test.js
+++ b/test/e2e-api/admin/posts.test.js
@@ -22,6 +22,11 @@ describe('Posts API', function () {
          * can't be overwritten after an email record is created.
          */
         await localUtils.doAuth(request, 'users:extra', 'posts', 'emails', 'newsletters', 'members');
+
+        // Assign a newsletter to one of the posts
+        const newsletterId = testUtils.DataGenerator.Content.newsletters[0].id;
+        const postId = testUtils.DataGenerator.Content.posts[0].id;
+        await models.Post.edit({newsletter_id: newsletterId}, {id: postId});
     });
 
     afterEach(function () {
@@ -60,6 +65,14 @@ describe('Posts API', function () {
         jsonResponse.posts[2].authors.length.should.eql(1);
         jsonResponse.posts[2].tags[0].url.should.eql(`${config.get('url')}/tag/getting-started/`);
         jsonResponse.posts[2].authors[0].url.should.eql(`${config.get('url')}/author/ghost/`);
+
+        // Check if the newsletter relation is loaded by default and newsletter_id is not returned
+        jsonResponse.posts[12].id.should.eql(testUtils.DataGenerator.Content.posts[0].id);
+        jsonResponse.posts[12].newsletter.id.should.eql(testUtils.DataGenerator.Content.newsletters[0].id);
+        should.not.exist(jsonResponse.posts[12].newsletter_id);
+
+        should(jsonResponse.posts[0].newsletter).be.null();
+        should.not.exist(jsonResponse.posts[0].newsletter_id);
     });
 
     it('Can retrieve multiple post formats', async function () {
@@ -98,7 +111,7 @@ describe('Posts API', function () {
             jsonResponse.posts[0],
             'post',
             null,
-            ['authors', 'primary_author', 'email', 'tiers']
+            ['authors', 'primary_author', 'email', 'tiers', 'newsletter']
         );
 
         localUtils.API.checkResponse(jsonResponse.meta.pagination, 'pagination');
@@ -162,6 +175,33 @@ describe('Posts API', function () {
         _.isBoolean(jsonResponse.posts[0].featured).should.eql(true);
 
         testUtils.API.isISO8601(jsonResponse.posts[0].created_at).should.be.true();
+
+        // Check if the newsletter relation is loaded by default and newsletter_id is not returned
+        jsonResponse.posts[0].newsletter.id.should.eql(testUtils.DataGenerator.Content.newsletters[0].id);
+        should.not.exist(jsonResponse.posts[0].newsletter_id);
+    });
+
+    it('Can request a post by id without newsletter', async function () {
+        const res = await request.get(localUtils.API.getApiQuery('posts/' + testUtils.DataGenerator.Content.posts[1].id + '/'))
+            .set('Origin', config.get('url'))
+            .expect('Content-Type', /json/)
+            .expect('Cache-Control', testUtils.cacheRules.private)
+            .expect(200);
+
+        should.not.exist(res.headers['x-cache-invalidate']);
+        const jsonResponse = res.body;
+        should.exist(jsonResponse);
+        should.exist(jsonResponse.posts);
+        localUtils.API.checkResponse(jsonResponse.posts[0], 'post');
+        jsonResponse.posts[0].id.should.equal(testUtils.DataGenerator.Content.posts[1].id);
+
+        _.isBoolean(jsonResponse.posts[0].featured).should.eql(true);
+
+        testUtils.API.isISO8601(jsonResponse.posts[0].created_at).should.be.true();
+
+        // Newsletter should be returned as null
+        should(jsonResponse.posts[0].newsletter).be.null();
+        should.not.exist(jsonResponse.posts[0].newsletter_id);
     });
 
     it('Can retrieve a post by slug', async function () {
@@ -179,11 +219,15 @@ describe('Posts API', function () {
         jsonResponse.posts[0].slug.should.equal('welcome');
 
         _.isBoolean(jsonResponse.posts[0].featured).should.eql(true);
+
+        // Newsletter should be returned as null
+        should(jsonResponse.posts[0].newsletter).be.null();
+        should.not.exist(jsonResponse.posts[0].newsletter_id);
     });
 
     it('Can include relations for a single post', async function () {
         const res = await request
-            .get(localUtils.API.getApiQuery('posts/' + testUtils.DataGenerator.Content.posts[0].id + '/?include=authors,tags,email,tiers'))
+            .get(localUtils.API.getApiQuery('posts/' + testUtils.DataGenerator.Content.posts[0].id + '/?include=authors,tags,email,tiers,newsletter'))
             .set('Origin', config.get('url'))
             .expect('Content-Type', /json/)
             .expect('Cache-Control', testUtils.cacheRules.private)
@@ -204,6 +248,9 @@ describe('Posts API', function () {
 
         jsonResponse.posts[0].email.should.be.an.Object();
         localUtils.API.checkResponse(jsonResponse.posts[0].email, 'email');
+
+        jsonResponse.posts[0].newsletter.id.should.eql(testUtils.DataGenerator.Content.newsletters[0].id);
+        should.not.exist(jsonResponse.posts[0].newsletter_id);
     });
 
     it('Can add a post', async function () {
@@ -235,6 +282,10 @@ describe('Posts API', function () {
         should.exist(res.headers.location);
         res.headers.location.should.equal(`http://127.0.0.1:2369${localUtils.API.getApiQuery('posts/')}${res.body.posts[0].id}/`);
 
+        // Newsletter should be returned as null
+        should(res.body.posts[0].newsletter).be.null();
+        should.not.exist(res.body.posts[0].newsletter_id);
+
         const model = await models.Post.findOne({
             id: res.body.posts[0].id,
             status: 'draft'
@@ -352,6 +403,10 @@ describe('Posts API', function () {
             .expect(200);
 
         res2.headers['x-cache-invalidate'].should.eql('/p/' + res2.body.posts[0].uuid + '/');
+
+        // Newsletter should be returned as null
+        should(res2.body.posts[0].newsletter).be.null();
+        should.not.exist(res2.body.posts[0].newsletter_id);
     });
 
     it('Can update and force re-render', async function () {
@@ -422,12 +477,18 @@ describe('Posts API', function () {
         res2.body.posts[0].status.should.eql('draft');
     });
 
-    it('Can\'t change the newsletter_id of a post from the post body', async function () {
+    it(`Can't change the newsletter_id of a post from the post body`, async function () {
         const post = {
             newsletter_id: testUtils.DataGenerator.Content.newsletters[0].id
         };
 
-        const postId = testUtils.DataGenerator.Content.posts[0].id;
+        const postId = testUtils.DataGenerator.Content.posts[2].id;
+
+        const modelBefore = await models.Post.findOne({
+            id: postId
+        }, testUtils.context.internal);
+
+        should(modelBefore.get('newsletter_id')).eql(null, 'This test requires the initial post to not have a newsletter');
 
         const res = await request
             .get(localUtils.API.getApiQuery(`posts/${postId}/?`))
@@ -436,7 +497,44 @@ describe('Posts API', function () {
 
         post.updated_at = res.body.posts[0].updated_at;
 
-        await request
+        const res2 = await request
+            .put(localUtils.API.getApiQuery('posts/' + postId + '/'))
+            .set('Origin', config.get('url'))
+            .send({posts: [post]})
+            .expect('Content-Type', /json/)
+            .expect('Cache-Control', testUtils.cacheRules.private)
+            .expect(422);
+
+        const model = await models.Post.findOne({
+            id: postId
+        }, testUtils.context.internal);
+
+        should(model.get('newsletter_id')).eql(null);
+    });
+
+    it(`Can't change the newsletter of a post from the post body`, async function () {
+        const post = {
+            newsletter: {
+                id: testUtils.DataGenerator.Content.newsletters[0].id
+            }
+        };
+
+        const postId = testUtils.DataGenerator.Content.posts[2].id;
+
+        const modelBefore = await models.Post.findOne({
+            id: postId
+        }, testUtils.context.internal);
+
+        should(modelBefore.get('newsletter_id')).eql(null, 'This test requires the initial post to not have a newsletter');
+
+        const res = await request
+            .get(localUtils.API.getApiQuery(`posts/${postId}/?`))
+            .set('Origin', config.get('url'))
+            .expect(200);
+
+        post.updated_at = res.body.posts[0].updated_at;
+
+        const res2 = await request
             .put(localUtils.API.getApiQuery('posts/' + postId + '/'))
             .set('Origin', config.get('url'))
             .send({posts: [post]})
@@ -451,6 +549,45 @@ describe('Posts API', function () {
         should(model.get('newsletter_id')).eql(null);
     });
 
+    it('Cannot change the newsletter via body when adding', async function () {
+        const post = {
+            title: 'My newsletter post',
+            status: 'draft',
+            feature_image_alt: 'Testing newsletter',
+            feature_image_caption: 'Testing <b>feature image caption</b>',
+            mobiledoc: testUtils.DataGenerator.markdownToMobiledoc('my post'),
+            created_at: moment().subtract(2, 'days').toDate(),
+            updated_at: moment().subtract(2, 'days').toDate(),
+            created_by: ObjectId().toHexString(),
+            updated_by: ObjectId().toHexString(),
+            newsletter: {
+                // This should be ignored, the default one should be used instead
+                id: testUtils.DataGenerator.Content.newsletters[0].id
+            }
+        };
+
+        const res = await request.post(localUtils.API.getApiQuery('posts'))
+            .set('Origin', config.get('url'))
+            .send({posts: [post]})
+            .expect('Content-Type', /json/)
+            .expect('Cache-Control', testUtils.cacheRules.private)
+            .expect(201);
+
+        // Check that the default newsletter is used instead of the one in body (not allowed)
+        should(res.body.posts[0].status).eql('draft');
+        should(res.body.posts[0].newsletter).eql(null);
+        should.not.exist(res.body.posts[0].newsletter_id);
+
+        const id = res.body.posts[0].id;
+
+        const model = await models.Post.findOne({
+            id,
+            status: 'draft' // Fix for default filter
+        }, testUtils.context.internal);
+
+        should(model.get('newsletter_id')).eql(null);
+    });
+
     it('Can change the newsletter_id of a post when publishing', async function () {
         const post = {
             title: 'My newsletter_id post',
@@ -471,6 +608,10 @@ describe('Posts API', function () {
             .expect('Cache-Control', testUtils.cacheRules.private)
             .expect(201);
 
+        // Check newsletter relation is loaded, but null in response.
+        should(res.body.posts[0].newsletter).eql(null);
+        should.not.exist(res.body.posts[0].newsletter_id);
+
         const id = res.body.posts[0].id;
 
         const updatedPost = res.body.posts[0];
@@ -487,6 +628,10 @@ describe('Posts API', function () {
             .expect('Cache-Control', testUtils.cacheRules.private)
             .expect(200);
 
+        // Check newsletter relation is loaded in response
+        should(finalPost.body.posts[0].newsletter.id).eql(newsletterId);
+        should.not.exist(finalPost.body.posts[0].newsletter_id);
+
         const model = await models.Post.findOne({
             id
         }, testUtils.context.internal);
@@ -495,6 +640,8 @@ describe('Posts API', function () {
     });
 
     it('Defaults to the default newsletter when publishing without a newsletter_id', async function () {
+        const defaultNewsletter = await models.Newsletter.getDefaultNewsletter();
+
         const post = {
             title: 'My post without newsletter_id',
             status: 'draft',
@@ -521,7 +668,7 @@ describe('Posts API', function () {
             updated_at: res.body.posts[0].updated_at
         };
 
-        await request
+        const finalPost = await request
             .put(localUtils.API.getApiQuery('posts/' + id + '/?email_recipient_filter=all'))
             .set('Origin', config.get('url'))
             .send({posts: [updatedPost]})
@@ -529,14 +676,14 @@ describe('Posts API', function () {
             .expect('Cache-Control', testUtils.cacheRules.private)
             .expect(200);
 
+        // Check newsletter relation is loaded in response
+        should(finalPost.body.posts[0].newsletter.id).eql(defaultNewsletter.get('id'));
+        should.not.exist(finalPost.body.posts[0].newsletter_id);
+
         const model = await models.Post.findOne({
             id
         }, testUtils.context.internal);
 
-        const defaultNewsletter = await models.Newsletter.findOne({
-            sort_order: 0
-        }, testUtils.context.internal);
-
         should(model.get('newsletter_id')).eql(defaultNewsletter.get('id'));
     });
 
@@ -578,6 +725,10 @@ describe('Posts API', function () {
             .expect('Cache-Control', testUtils.cacheRules.private)
             .expect(200);
 
+        // Check newsletter relation is loaded in response
+        should(res2.body.posts[0].newsletter.id).eql(newsletterId);
+        should.not.exist(res2.body.posts[0].newsletter_id);
+
         model = await models.Post.findOne({
             id: id,
             status: 'published'
@@ -597,6 +748,10 @@ describe('Posts API', function () {
             .expect('Cache-Control', testUtils.cacheRules.private)
             .expect(200);
 
+        // Check newsletter relation is loaded in response
+        should(res3.body.posts[0].newsletter).eql(null);
+        should.not.exist(res3.body.posts[0].newsletter_id);
+
         model = await models.Post.findOne({
             id: id,
             status: 'draft'
@@ -611,7 +766,7 @@ describe('Posts API', function () {
             updated_at: res3.body.posts[0].updated_at
         };
 
-        await request
+        const res4 = await request
             .put(localUtils.API.getApiQuery('posts/' + id + '/?email_recipient_filter=all&newsletter_id=' + newsletterId2))
             .set('Origin', config.get('url'))
             .send({posts: [republished]})
@@ -619,6 +774,11 @@ describe('Posts API', function () {
             .expect('Cache-Control', testUtils.cacheRules.private)
             .expect(200);
 
+        // Check newsletter relation is loaded in response
+        // + did update the newsletter id
+        should(res4.body.posts[0].newsletter.id).eql(newsletterId2);
+        should.not.exist(res4.body.posts[0].newsletter_id);
+
         model = await models.Post.findOne({
             id: id,
             status: 'published'
@@ -626,7 +786,7 @@ describe('Posts API', function () {
         should(model.get('newsletter_id')).eql(newsletterId2);
 
         // Should not change if status remains published
-        await request
+        const res5 = await request
             .put(localUtils.API.getApiQuery('posts/' + id + '/?email_recipient_filter=all&newsletter_id=' + newsletterId))
             .set('Origin', config.get('url'))
             .send({posts: [republished]})
@@ -634,6 +794,11 @@ describe('Posts API', function () {
             .expect('Cache-Control', testUtils.cacheRules.private)
             .expect(200);
 
+        // Check newsletter relation is loaded in response
+        // + did not update the newsletter id
+        should(res5.body.posts[0].newsletter.id).eql(newsletterId2);
+        should.not.exist(res5.body.posts[0].newsletter_id);
+
         model = await models.Post.findOne({
             id: id,
             status: 'published'
diff --git a/test/e2e-api/admin/utils.js b/test/e2e-api/admin/utils.js
index aeee1f0e41..e06f5296c5 100644
--- a/test/e2e-api/admin/utils.js
+++ b/test/e2e-api/admin/utils.js
@@ -71,7 +71,7 @@ const expectedProperties = {
         'frontmatter',
         'email_only',
         'tiers',
-        'newsletter_id'
+        'newsletter'
     ],
 
     page: [
diff --git a/test/regression/api/admin/utils.js b/test/regression/api/admin/utils.js
index 0c81a74357..2e01e1fdd6 100644
--- a/test/regression/api/admin/utils.js
+++ b/test/regression/api/admin/utils.js
@@ -65,7 +65,7 @@ const expectedProperties = {
         'frontmatter',
         'email_only',
         'tiers',
-        'newsletter_id'
+        'newsletter'
     ],
     user: [
         'id',
diff --git a/yarn.lock b/yarn.lock
index c4608c58a5..f5e8ad83e1 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -1719,10 +1719,10 @@
   dependencies:
     "@tryghost/errors" "^1.2.1"
 
-"@tryghost/admin-api-schema@2.14.1":
-  version "2.14.1"
-  resolved "https://registry.yarnpkg.com/@tryghost/admin-api-schema/-/admin-api-schema-2.14.1.tgz#cc9485847671014b8e86b31b73ef7b96f7d22e83"
-  integrity sha512-VnrwzOlIjIIIYJi64iQzINMqBNQe1wbafe4yXQsL0leaHlBCrucEGYZVMpq25pISkBpCg8kGqYksuJdyfIQ4KA==
+"@tryghost/admin-api-schema@2.15.0":
+  version "2.15.0"
+  resolved "https://registry.yarnpkg.com/@tryghost/admin-api-schema/-/admin-api-schema-2.15.0.tgz#fb2a14af9dd5d418d3a2f5a7ce89b349ac092b2b"
+  integrity sha512-zh1aod5J3DwKuhiJ1JbkMT8ybs0DfHjs7ubwSc+GUkWaa6w8yyDFukzDC59P3PPV3DMTyKwzJlEqv2S86QQmOw==
   dependencies:
     "@tryghost/errors" "^1.0.0"
     lodash "^4.17.11"
@@ -1838,10 +1838,10 @@
   dependencies:
     long-timeout "^0.1.1"
 
-"@tryghost/color-utils@0.1.14":
-  version "0.1.14"
-  resolved "https://registry.yarnpkg.com/@tryghost/color-utils/-/color-utils-0.1.14.tgz#4b794bf79a1b813608725e0456b100f28f881e7d"
-  integrity sha512-3mIs+z3P+BR2nF4a6zCWN2GEeZlKwK6JfxEVhrmRcwsIvWVtpPaMAT4FjozuXlBz5JskIOLZDfXS6AcP+cc3nw==
+"@tryghost/color-utils@0.1.15":
+  version "0.1.15"
+  resolved "https://registry.yarnpkg.com/@tryghost/color-utils/-/color-utils-0.1.15.tgz#814f082ae4ee65c96b0429ab196af2772b7b25c4"
+  integrity sha512-LI8UKboknIwCewL3z1l5FS4V0W84Nfc4snA1n62F3a8casS6V+imtqdGb1qgu+Y/0IhVxLbMSM5/qOpJzZoPiQ==
   dependencies:
     color "^3.2.1"
 
@@ -1971,10 +1971,10 @@
     cookiejar "^2.1.3"
     reqresnext "^1.7.0"
 
-"@tryghost/helpers@1.1.65":
-  version "1.1.65"
-  resolved "https://registry.yarnpkg.com/@tryghost/helpers/-/helpers-1.1.65.tgz#02d17739c35d50bf1d7fd1cc12ff5c0ed0f40efd"
-  integrity sha512-IOtGa638BtIq4W0k/D9+RbawRjMrYtvwHdfS+GdJaMHAQQWa68jxTQfHBIinPLk5n4nCJHNaXScSiLbakQ/j0w==
+"@tryghost/helpers@1.1.66":
+  version "1.1.66"
+  resolved "https://registry.yarnpkg.com/@tryghost/helpers/-/helpers-1.1.66.tgz#dcd485975209e4aff78d4f0ae598855852c7b9e1"
+  integrity sha512-GizIDYroVSLKxSTTMZ2OSrljB5eze59ZkkJM8pvabedrdV+km2uL0dubOiOtpJKcXzNj6UU7KEhoip6f4BoAnA==
   dependencies:
     lodash-es "^4.17.11"