diff --git a/services/mailer/mail.go b/services/mailer/mail.go
index 86bd40ff29..b04925881d 100644
--- a/services/mailer/mail.go
+++ b/services/mailer/mail.go
@@ -517,7 +517,7 @@ func actionToTemplate(issue *issues_model.Issue, actionType activities_model.Act
 			case issues_model.ReviewTypeReject:
 				name = "reject"
 			default:
-				name = "review"
+				name = "review" // TODO: there is no activities_model.Action* when sending a review comment, this is deadcode and should be removed
 			}
 		case issues_model.CommentTypeCode:
 			name = "code"
diff --git a/services/mailer/mail_admin_new_user.go b/services/mailer/mail_admin_new_user.go
index aa0571e57c..54287b1b7e 100644
--- a/services/mailer/mail_admin_new_user.go
+++ b/services/mailer/mail_admin_new_user.go
@@ -19,14 +19,11 @@ const (
 	tplNewUserMail base.TplName = "notify/admin_new_user"
 )
 
-var sa = SendAsync
-
 // MailNewUser sends notification emails on new user registrations to all admins
 func MailNewUser(ctx context.Context, u *user_model.User) {
 	if !setting.Admin.SendNotificationEmailOnNewUser {
 		return
 	}
-
 	if setting.MailService == nil {
 		// No mail service configured
 		return
@@ -77,5 +74,5 @@ func mailNewUser(ctx context.Context, u *user_model.User, lang string, tos []str
 		msg.Info = subject
 		msgs = append(msgs, msg)
 	}
-	sa(msgs...)
+	SendAsync(msgs...)
 }
diff --git a/services/mailer/mail_admin_new_user_test.go b/services/mailer/mail_admin_new_user_test.go
index b89d888ee1..603a8b95c9 100644
--- a/services/mailer/mail_admin_new_user_test.go
+++ b/services/mailer/mail_admin_new_user_test.go
@@ -11,10 +11,12 @@ import (
 	"code.gitea.io/gitea/models/db"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/setting"
-	"code.gitea.io/gitea/modules/translation"
+	"code.gitea.io/gitea/modules/test"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+
+	_ "github.com/mattn/go-sqlite3"
 )
 
 func getTestUsers(t *testing.T) []*user_model.User {
@@ -45,54 +47,35 @@ func cleanUpUsers(ctx context.Context, users []*user_model.User) {
 }
 
 func TestAdminNotificationMail_test(t *testing.T) {
-	translation.InitLocales(context.Background())
-	locale := translation.NewLocale("")
-	key := "mail.admin.new_user.user_info"
-	translatedKey := locale.Tr(key)
-	require.NotEqualValues(t, key, translatedKey)
-
-	mailService := setting.Mailer{
-		From:     "test@example.com",
-		Protocol: "dummy",
-	}
-
-	setting.MailService = &mailService
-
-	// test with SEND_NOTIFICATION_EMAIL_ON_NEW_USER enabled
-	setting.Admin.SendNotificationEmailOnNewUser = true
-
 	ctx := context.Background()
-	NewContext(ctx)
 
 	users := getTestUsers(t)
-	oldSendAsync := sa
-	defer func() {
-		sa = oldSendAsync
-		cleanUpUsers(ctx, users)
-	}()
 
-	called := false
-	sa = func(msgs ...*Message) {
-		assert.Equal(t, len(msgs), 1, "Test provides only one admin user, so only one email must be sent")
-		assert.Equal(t, msgs[0].To, users[0].Email, "checks if the recipient is the admin of the instance")
-		manageUserURL := setting.AppURL + "admin/users/" + strconv.FormatInt(users[1].ID, 10)
-		assert.Contains(t, msgs[0].Body, manageUserURL)
-		assert.Contains(t, msgs[0].Body, users[1].HTMLURL())
-		assert.Contains(t, msgs[0].Body, translatedKey, "the .Locale translates to nothing")
-		assert.Contains(t, msgs[0].Body, users[1].Name, "user name of the newly created user")
-		for _, untranslated := range []string{"mail.admin", "admin.users"} {
-			assert.NotContains(t, msgs[0].Body, untranslated, "this is an untranslated placeholder prefix")
-		}
-		called = true
-	}
-	MailNewUser(ctx, users[1])
-	assert.True(t, called)
+	t.Run("SendNotificationEmailOnNewUser_true", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.Admin.SendNotificationEmailOnNewUser, true)()
 
-	// test with SEND_NOTIFICATION_EMAIL_ON_NEW_USER disabled; emails shouldn't be sent
-	setting.Admin.SendNotificationEmailOnNewUser = false
-	sa = func(msgs ...*Message) {
-		assert.Equal(t, 1, 0, "this shouldn't execute. MailNewUser must exit early since SEND_NOTIFICATION_EMAIL_ON_NEW_USER is disabled")
-	}
+		called := false
+		defer mockMailSettings(func(msgs ...*Message) {
+			assert.Equal(t, len(msgs), 1, "Test provides only one admin user, so only one email must be sent")
+			assert.Equal(t, msgs[0].To, users[0].Email, "checks if the recipient is the admin of the instance")
+			manageUserURL := setting.AppURL + "admin/users/" + strconv.FormatInt(users[1].ID, 10)
+			assert.Contains(t, msgs[0].Body, manageUserURL)
+			assert.Contains(t, msgs[0].Body, users[1].HTMLURL())
+			assert.Contains(t, msgs[0].Body, users[1].Name, "user name of the newly created user")
+			assertTranslatedLocale(t, msgs[0].Body, "mail.admin", "admin.users")
+			called = true
+		})()
+		MailNewUser(ctx, users[1])
+		assert.True(t, called)
+	})
 
-	MailNewUser(ctx, users[1])
+	t.Run("SendNotificationEmailOnNewUser_false", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.Admin.SendNotificationEmailOnNewUser, false)()
+		defer mockMailSettings(func(msgs ...*Message) {
+			assert.Equal(t, 1, 0, "this shouldn't execute. MailNewUser must exit early since SEND_NOTIFICATION_EMAIL_ON_NEW_USER is disabled")
+		})()
+		MailNewUser(ctx, users[1])
+	})
+
+	cleanUpUsers(ctx, users)
 }
diff --git a/services/mailer/mail_test.go b/services/mailer/mail_test.go
index 528b11f16b..ed972dbb99 100644
--- a/services/mailer/mail_test.go
+++ b/services/mailer/mail_test.go
@@ -23,6 +23,7 @@ import (
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/markup"
 	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/test"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -51,12 +52,6 @@ const bodyTpl = `
 
 func prepareMailerTest(t *testing.T) (doer *user_model.User, repo *repo_model.Repository, issue *issues_model.Issue, comment *issues_model.Comment) {
 	assert.NoError(t, unittest.PrepareTestDatabase())
-	mailService := setting.Mailer{
-		From: "test@gitea.com",
-	}
-
-	setting.MailService = &mailService
-	setting.Domain = "localhost"
 
 	doer = unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
 	repo = unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 1, Owner: doer})
@@ -67,6 +62,7 @@ func prepareMailerTest(t *testing.T) (doer *user_model.User, repo *repo_model.Re
 }
 
 func TestComposeIssueCommentMessage(t *testing.T) {
+	defer mockMailSettings(nil)()
 	doer, _, issue, comment := prepareMailerTest(t)
 
 	markup.Init(&markup.ProcessorHelper{
@@ -75,8 +71,7 @@ func TestComposeIssueCommentMessage(t *testing.T) {
 		},
 	})
 
-	setting.IncomingEmail.Enabled = true
-	defer func() { setting.IncomingEmail.Enabled = false }()
+	defer test.MockVariableValue(&setting.IncomingEmail.Enabled, true)()
 
 	subjectTemplates = texttmpl.Must(texttmpl.New("issue/comment").Parse(subjectTpl))
 	bodyTemplates = template.Must(template.New("issue/comment").Parse(bodyTpl))
@@ -122,11 +117,9 @@ func TestComposeIssueCommentMessage(t *testing.T) {
 }
 
 func TestComposeIssueMessage(t *testing.T) {
+	defer mockMailSettings(nil)()
 	doer, _, issue, _ := prepareMailerTest(t)
 
-	subjectTemplates = texttmpl.Must(texttmpl.New("issue/new").Parse(subjectTpl))
-	bodyTemplates = template.Must(template.New("issue/new").Parse(bodyTpl))
-
 	recipients := []*user_model.User{{Name: "Test", Email: "test@gitea.com"}, {Name: "Test2", Email: "test2@gitea.com"}}
 	msgs, err := composeIssueCommentMessages(&mailCommentContext{
 		Context: context.TODO(), // TODO: use a correct context
@@ -144,7 +137,7 @@ func TestComposeIssueMessage(t *testing.T) {
 	references := gomailMsg.GetHeader("References")
 
 	assert.Len(t, mailto, 1, "exactly one recipient is expected in the To field")
-	assert.Equal(t, "[user2/repo1] @user2 #1 - issue1", subject[0])
+	assert.Equal(t, "[user2/repo1] issue1 (#1)", subject[0])
 	assert.Equal(t, "<user2/repo1/issues/1@localhost>", inReplyTo[0], "In-Reply-To header doesn't match")
 	assert.Equal(t, "<user2/repo1/issues/1@localhost>", references[0], "References header doesn't match")
 	assert.Equal(t, "<user2/repo1/issues/1@localhost>", messageID[0], "Message-ID header doesn't match")
@@ -152,7 +145,103 @@ func TestComposeIssueMessage(t *testing.T) {
 	assert.Len(t, gomailMsg.GetHeader("List-Unsubscribe"), 1) // url without mailto
 }
 
+func TestMailerIssueTemplate(t *testing.T) {
+	defer mockMailSettings(nil)()
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	doer := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
+
+	expect := func(t *testing.T, msg *Message, issue *issues_model.Issue, expected ...string) {
+		subject := msg.ToMessage().GetHeader("Subject")
+		msgbuf := new(bytes.Buffer)
+		_, _ = msg.ToMessage().WriteTo(msgbuf)
+		wholemsg := msgbuf.String()
+		assert.Contains(t, subject[0], fallbackMailSubject(issue))
+		for _, s := range expected {
+			assert.Contains(t, wholemsg, s)
+		}
+		assertTranslatedLocale(t, wholemsg, "mail.issue")
+	}
+
+	testCompose := func(t *testing.T, ctx *mailCommentContext) *Message {
+		t.Helper()
+		recipients := []*user_model.User{{Name: "Test", Email: "test@gitea.com"}}
+
+		ctx.Context = context.Background()
+		fromMention := false
+		msgs, err := composeIssueCommentMessages(ctx, "en-US", recipients, fromMention, "TestMailerIssueTemplate")
+		assert.NoError(t, err)
+		assert.Len(t, msgs, 1)
+		return msgs[0]
+	}
+
+	issue := unittest.AssertExistsAndLoadBean(t, &issues_model.Issue{ID: 1})
+	assert.NoError(t, issue.LoadRepo(db.DefaultContext))
+
+	msg := testCompose(t, &mailCommentContext{
+		Issue: issue, Doer: doer, ActionType: activities_model.ActionCreateIssue,
+		Content: issue.Content,
+	})
+	expect(t, msg, issue, issue.Content)
+
+	comment := unittest.AssertExistsAndLoadBean(t, &issues_model.Comment{ID: 2, Issue: issue})
+
+	msg = testCompose(t, &mailCommentContext{
+		Issue: issue, Doer: doer, ActionType: activities_model.ActionCommentIssue,
+		Content: comment.Content, Comment: comment,
+	})
+	expect(t, msg, issue, comment.Content)
+
+	msg = testCompose(t, &mailCommentContext{
+		Issue: issue, Doer: doer, ActionType: activities_model.ActionCloseIssue,
+		Content: comment.Content, Comment: comment,
+	})
+	expect(t, msg, issue, comment.Content)
+
+	msg = testCompose(t, &mailCommentContext{
+		Issue: issue, Doer: doer, ActionType: activities_model.ActionReopenIssue,
+		Content: comment.Content, Comment: comment,
+	})
+	expect(t, msg, issue, comment.Content)
+
+	pull := unittest.AssertExistsAndLoadBean(t, &issues_model.Issue{ID: 2})
+	assert.NoError(t, pull.LoadAttributes(db.DefaultContext))
+	pullComment := unittest.AssertExistsAndLoadBean(t, &issues_model.Comment{ID: 4, Issue: pull})
+
+	msg = testCompose(t, &mailCommentContext{
+		Issue: pull, Doer: doer, ActionType: activities_model.ActionCommentPull,
+		Content: pullComment.Content, Comment: pullComment,
+	})
+	expect(t, msg, pull, pullComment.Content)
+
+	msg = testCompose(t, &mailCommentContext{
+		Issue: pull, Doer: doer, ActionType: activities_model.ActionMergePullRequest,
+		Content: pullComment.Content, Comment: pullComment,
+	})
+	expect(t, msg, pull, pullComment.Content, pull.PullRequest.BaseBranch)
+
+	reviewComment := unittest.AssertExistsAndLoadBean(t, &issues_model.Comment{ID: 9})
+	assert.NoError(t, reviewComment.LoadReview(db.DefaultContext))
+
+	approveComment := reviewComment
+	approveComment.Review.Type = issues_model.ReviewTypeApprove
+	msg = testCompose(t, &mailCommentContext{
+		Issue: pull, Doer: doer, ActionType: activities_model.ActionApprovePullRequest,
+		Content: approveComment.Content, Comment: approveComment,
+	})
+	expect(t, msg, pull, approveComment.Content)
+
+	rejectComment := reviewComment
+	rejectComment.Review.Type = issues_model.ReviewTypeReject
+	msg = testCompose(t, &mailCommentContext{
+		Issue: pull, Doer: doer, ActionType: activities_model.ActionRejectPullRequest,
+		Content: rejectComment.Content, Comment: rejectComment,
+	})
+	expect(t, msg, pull, rejectComment.Content)
+}
+
 func TestTemplateSelection(t *testing.T) {
+	defer mockMailSettings(nil)()
 	doer, repo, issue, comment := prepareMailerTest(t)
 	recipients := []*user_model.User{{Name: "Test", Email: "test@gitea.com"}}
 
@@ -207,6 +296,7 @@ func TestTemplateSelection(t *testing.T) {
 }
 
 func TestTemplateServices(t *testing.T) {
+	defer mockMailSettings(nil)()
 	doer, _, issue, comment := prepareMailerTest(t)
 	assert.NoError(t, issue.LoadRepo(db.DefaultContext))
 
@@ -259,6 +349,7 @@ func testComposeIssueCommentMessage(t *testing.T, ctx *mailCommentContext, recip
 }
 
 func TestGenerateAdditionalHeaders(t *testing.T) {
+	defer mockMailSettings(nil)()
 	doer, _, issue, _ := prepareMailerTest(t)
 
 	ctx := &mailCommentContext{Context: context.TODO() /* TODO: use a correct context */, Issue: issue, Doer: doer}
@@ -288,6 +379,7 @@ func TestGenerateAdditionalHeaders(t *testing.T) {
 }
 
 func Test_createReference(t *testing.T) {
+	defer mockMailSettings(nil)()
 	_, _, issue, comment := prepareMailerTest(t)
 	_, _, pullIssue, _ := prepareMailerTest(t)
 	pullIssue.IsPull = true
diff --git a/services/mailer/mailer.go b/services/mailer/mailer.go
index 5e8e3dbb38..a9316cca76 100644
--- a/services/mailer/mailer.go
+++ b/services/mailer/mailer.go
@@ -365,10 +365,8 @@ func (s *sendmailSender) Send(from string, to []string, msg io.WriterTo) error {
 	return waitError
 }
 
-// Sender sendmail mail sender
 type dummySender struct{}
 
-// Send send email
 func (s *dummySender) Send(from string, to []string, msg io.WriterTo) error {
 	buf := bytes.Buffer{}
 	if _, err := msg.WriteTo(&buf); err != nil {
diff --git a/services/mailer/main_test.go b/services/mailer/main_test.go
index f803c736ca..399d05ac7b 100644
--- a/services/mailer/main_test.go
+++ b/services/mailer/main_test.go
@@ -4,13 +4,45 @@
 package mailer
 
 import (
+	"context"
 	"testing"
 
 	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/templates"
+	"code.gitea.io/gitea/modules/test"
+	"code.gitea.io/gitea/modules/translation"
 
 	_ "code.gitea.io/gitea/models/actions"
+
+	"github.com/stretchr/testify/assert"
 )
 
 func TestMain(m *testing.M) {
 	unittest.MainTest(m)
 }
+
+func assertTranslatedLocale(t *testing.T, message string, prefixes ...string) {
+	t.Helper()
+	for _, prefix := range prefixes {
+		assert.NotContains(t, message, prefix, "there is an untranslated locale prefix")
+	}
+}
+
+func mockMailSettings(send func(msgs ...*Message)) func() {
+	translation.InitLocales(context.Background())
+	subjectTemplates, bodyTemplates = templates.Mailer(context.Background())
+	mailService := setting.Mailer{
+		From: "test@gitea.com",
+	}
+	cleanups := []func(){
+		test.MockVariableValue(&setting.MailService, &mailService),
+		test.MockVariableValue(&setting.Domain, "localhost"),
+		test.MockVariableValue(&SendAsync, send),
+	}
+	return func() {
+		for _, cleanup := range cleanups {
+			cleanup()
+		}
+	}
+}