diff --git a/modules/templates/mailer.go b/modules/templates/mailer.go
index 310d645328..c43b760777 100644
--- a/modules/templates/mailer.go
+++ b/modules/templates/mailer.go
@@ -9,6 +9,7 @@ import (
 	"html/template"
 	"regexp"
 	"strings"
+	"sync/atomic"
 	texttmpl "text/template"
 
 	"code.gitea.io/gitea/modules/log"
@@ -16,6 +17,12 @@ import (
 	"code.gitea.io/gitea/modules/util"
 )
 
+type MailTemplates struct {
+	TemplateNames    []string
+	BodyTemplates    *template.Template
+	SubjectTemplates *texttmpl.Template
+}
+
 var mailSubjectSplit = regexp.MustCompile(`(?m)^-{3,}\s*$`)
 
 // mailSubjectTextFuncMap returns functions for injecting to text templates, it's only used for mail subject
@@ -52,16 +59,17 @@ func buildSubjectBodyTemplate(stpl *texttmpl.Template, btpl *template.Template,
 	return nil
 }
 
-// Mailer provides the templates required for sending notification mails.
-func Mailer(ctx context.Context) (*texttmpl.Template, *template.Template) {
-	subjectTemplates := texttmpl.New("")
-	bodyTemplates := template.New("")
-
-	subjectTemplates.Funcs(mailSubjectTextFuncMap())
-	bodyTemplates.Funcs(NewFuncMap())
-
+// LoadMailTemplates provides the templates required for sending notification mails.
+func LoadMailTemplates(ctx context.Context, loadedTemplates *atomic.Pointer[MailTemplates]) {
 	assetFS := AssetFS()
 	refreshTemplates := func(firstRun bool) {
+		var templateNames []string
+		subjectTemplates := texttmpl.New("")
+		bodyTemplates := template.New("")
+
+		subjectTemplates.Funcs(mailSubjectTextFuncMap())
+		bodyTemplates.Funcs(NewFuncMap())
+
 		if !firstRun {
 			log.Trace("Reloading mail templates")
 		}
@@ -81,6 +89,7 @@ func Mailer(ctx context.Context) (*texttmpl.Template, *template.Template) {
 			if firstRun {
 				log.Trace("Adding mail template %s: %s by %s", tmplName, assetPath, layerName)
 			}
+			templateNames = append(templateNames, tmplName)
 			if err = buildSubjectBodyTemplate(subjectTemplates, bodyTemplates, tmplName, content); err != nil {
 				if firstRun {
 					log.Fatal("Failed to parse mail template, err: %v", err)
@@ -88,6 +97,12 @@ func Mailer(ctx context.Context) (*texttmpl.Template, *template.Template) {
 				log.Error("Failed to parse mail template, err: %v", err)
 			}
 		}
+		loaded := &MailTemplates{
+			TemplateNames:    templateNames,
+			BodyTemplates:    bodyTemplates,
+			SubjectTemplates: subjectTemplates,
+		}
+		loadedTemplates.Store(loaded)
 	}
 
 	refreshTemplates(true)
@@ -99,6 +114,4 @@ func Mailer(ctx context.Context) (*texttmpl.Template, *template.Template) {
 			refreshTemplates(false)
 		})
 	}
-
-	return subjectTemplates, bodyTemplates
 }
diff --git a/routers/web/devtest/mail_preview.go b/routers/web/devtest/mail_preview.go
new file mode 100644
index 0000000000..79dd441eab
--- /dev/null
+++ b/routers/web/devtest/mail_preview.go
@@ -0,0 +1,58 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package devtest
+
+import (
+	"net/http"
+	"strings"
+
+	"code.gitea.io/gitea/modules/templates"
+	"code.gitea.io/gitea/services/context"
+	"code.gitea.io/gitea/services/mailer"
+
+	"gopkg.in/yaml.v3"
+)
+
+func MailPreviewRender(ctx *context.Context) {
+	tmplName := ctx.PathParam("*")
+	mockDataContent, err := templates.AssetFS().ReadFile("mail/" + tmplName + ".mock.yml")
+	mockData := map[string]any{}
+	if err == nil {
+		err = yaml.Unmarshal(mockDataContent, &mockData)
+		if err != nil {
+			http.Error(ctx.Resp, "Failed to parse mock data: "+err.Error(), http.StatusInternalServerError)
+			return
+		}
+	}
+	mockData["locale"] = ctx.Locale
+	err = mailer.LoadedTemplates().BodyTemplates.ExecuteTemplate(ctx.Resp, tmplName, mockData)
+	if err != nil {
+		_, _ = ctx.Resp.Write([]byte(err.Error()))
+	}
+}
+
+func prepareMailPreviewRender(ctx *context.Context, tmplName string) {
+	tmplSubject := mailer.LoadedTemplates().SubjectTemplates.Lookup(tmplName)
+	if tmplSubject == nil {
+		ctx.Data["RenderMailSubject"] = "default subject"
+	} else {
+		var buf strings.Builder
+		err := tmplSubject.Execute(&buf, nil)
+		if err != nil {
+			ctx.Data["RenderMailSubject"] = err.Error()
+		} else {
+			ctx.Data["RenderMailSubject"] = buf.String()
+		}
+	}
+	ctx.Data["RenderMailTemplateName"] = tmplName
+}
+
+func MailPreview(ctx *context.Context) {
+	ctx.Data["MailTemplateNames"] = mailer.LoadedTemplates().TemplateNames
+	tmplName := ctx.FormString("tmpl")
+	if tmplName != "" {
+		prepareMailPreviewRender(ctx, tmplName)
+	}
+	ctx.HTML(http.StatusOK, "devtest/mail-preview")
+}
diff --git a/routers/web/web.go b/routers/web/web.go
index 66c3a2da09..ddea468d17 100644
--- a/routers/web/web.go
+++ b/routers/web/web.go
@@ -1659,6 +1659,8 @@ func registerWebRoutes(m *web.Router) {
 		m.Group("/devtest", func() {
 			m.Any("", devtest.List)
 			m.Any("/fetch-action-test", devtest.FetchActionTest)
+			m.Any("/mail-preview", devtest.MailPreview)
+			m.Any("/mail-preview/*", devtest.MailPreviewRender)
 			m.Any("/{sub}", devtest.TmplCommon)
 			m.Get("/repo-action-view/{run}/{job}", devtest.MockActionsView)
 			m.Post("/actions-mock/runs/{run}/jobs/{job}", web.Bind(actions.ViewRequest{}), devtest.MockActionsRunsJobs)
diff --git a/services/mailer/mail.go b/services/mailer/mail.go
index aa51cbdbcf..b7602e0321 100644
--- a/services/mailer/mail.go
+++ b/services/mailer/mail.go
@@ -15,7 +15,7 @@ import (
 	"mime"
 	"regexp"
 	"strings"
-	texttmpl "text/template"
+	"sync/atomic"
 
 	repo_model "code.gitea.io/gitea/models/repo"
 	user_model "code.gitea.io/gitea/models/user"
@@ -23,6 +23,7 @@ import (
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/storage"
+	"code.gitea.io/gitea/modules/templates"
 	"code.gitea.io/gitea/modules/typesniffer"
 	sender_service "code.gitea.io/gitea/services/mailer/sender"
 
@@ -31,11 +32,13 @@ import (
 
 const mailMaxSubjectRunes = 256 // There's no actual limit for subject in RFC 5322
 
-var (
-	bodyTemplates       *template.Template
-	subjectTemplates    *texttmpl.Template
-	subjectRemoveSpaces = regexp.MustCompile(`[\s]+`)
-)
+var loadedTemplates atomic.Pointer[templates.MailTemplates]
+
+var subjectRemoveSpaces = regexp.MustCompile(`[\s]+`)
+
+func LoadedTemplates() *templates.MailTemplates {
+	return loadedTemplates.Load()
+}
 
 // SendTestMail sends a test mail
 func SendTestMail(email string) error {
diff --git a/services/mailer/mail_issue_common.go b/services/mailer/mail_issue_common.go
index ebfd52162c..107f57772c 100644
--- a/services/mailer/mail_issue_common.go
+++ b/services/mailer/mail_issue_common.go
@@ -119,7 +119,7 @@ func composeIssueCommentMessages(ctx context.Context, comment *mailComment, lang
 	}
 
 	var mailSubject bytes.Buffer
-	if err := subjectTemplates.ExecuteTemplate(&mailSubject, tplName, mailMeta); err == nil {
+	if err := LoadedTemplates().SubjectTemplates.ExecuteTemplate(&mailSubject, tplName, mailMeta); err == nil {
 		subject = sanitizeSubject(mailSubject.String())
 		if subject == "" {
 			subject = fallback
@@ -134,7 +134,7 @@ func composeIssueCommentMessages(ctx context.Context, comment *mailComment, lang
 
 	var mailBody bytes.Buffer
 
-	if err := bodyTemplates.ExecuteTemplate(&mailBody, tplName, mailMeta); err != nil {
+	if err := LoadedTemplates().BodyTemplates.ExecuteTemplate(&mailBody, tplName, mailMeta); err != nil {
 		log.Error("ExecuteTemplate [%s]: %v", tplName+"/body", err)
 	}
 
@@ -260,14 +260,14 @@ func actionToTemplate(issue *issues_model.Issue, actionType activities_model.Act
 	}
 
 	template = typeName + "/" + name
-	ok := bodyTemplates.Lookup(template) != nil
+	ok := LoadedTemplates().BodyTemplates.Lookup(template) != nil
 	if !ok && typeName != "issue" {
 		template = "issue/" + name
-		ok = bodyTemplates.Lookup(template) != nil
+		ok = LoadedTemplates().BodyTemplates.Lookup(template) != nil
 	}
 	if !ok {
 		template = typeName + "/default"
-		ok = bodyTemplates.Lookup(template) != nil
+		ok = LoadedTemplates().BodyTemplates.Lookup(template) != nil
 	}
 	if !ok {
 		template = "issue/default"
diff --git a/services/mailer/mail_release.go b/services/mailer/mail_release.go
index bfff73c39c..fd97fb5312 100644
--- a/services/mailer/mail_release.go
+++ b/services/mailer/mail_release.go
@@ -79,7 +79,7 @@ func mailNewRelease(ctx context.Context, lang string, tos []*user_model.User, re
 
 	var mailBody bytes.Buffer
 
-	if err := bodyTemplates.ExecuteTemplate(&mailBody, string(tplNewReleaseMail), mailMeta); err != nil {
+	if err := LoadedTemplates().BodyTemplates.ExecuteTemplate(&mailBody, string(tplNewReleaseMail), mailMeta); err != nil {
 		log.Error("ExecuteTemplate [%s]: %v", string(tplNewReleaseMail)+"/body", err)
 		return
 	}
diff --git a/services/mailer/mail_repo.go b/services/mailer/mail_repo.go
index b6b2d5ca07..1ec7995ab9 100644
--- a/services/mailer/mail_repo.go
+++ b/services/mailer/mail_repo.go
@@ -78,7 +78,7 @@ func sendRepoTransferNotifyMailPerLang(lang string, newOwner, doer *user_model.U
 		"Destination": destination,
 	}
 
-	if err := bodyTemplates.ExecuteTemplate(&content, string(mailRepoTransferNotify), data); err != nil {
+	if err := LoadedTemplates().BodyTemplates.ExecuteTemplate(&content, string(mailRepoTransferNotify), data); err != nil {
 		return err
 	}
 
diff --git a/services/mailer/mail_team_invite.go b/services/mailer/mail_team_invite.go
index f4aa788dec..034dc14e3d 100644
--- a/services/mailer/mail_team_invite.go
+++ b/services/mailer/mail_team_invite.go
@@ -62,7 +62,7 @@ func MailTeamInvite(ctx context.Context, inviter *user_model.User, team *org_mod
 	}
 
 	var mailBody bytes.Buffer
-	if err := bodyTemplates.ExecuteTemplate(&mailBody, string(tplTeamInviteMail), mailMeta); err != nil {
+	if err := LoadedTemplates().BodyTemplates.ExecuteTemplate(&mailBody, string(tplTeamInviteMail), mailMeta); err != nil {
 		log.Error("ExecuteTemplate [%s]: %v", string(tplTeamInviteMail)+"/body", err)
 		return err
 	}
diff --git a/services/mailer/mail_test.go b/services/mailer/mail_test.go
index b15949f352..3996796beb 100644
--- a/services/mailer/mail_test.go
+++ b/services/mailer/mail_test.go
@@ -25,6 +25,7 @@ import (
 	"code.gitea.io/gitea/modules/markup"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/storage"
+	"code.gitea.io/gitea/modules/templates"
 	"code.gitea.io/gitea/modules/test"
 	"code.gitea.io/gitea/services/attachment"
 	sender_service "code.gitea.io/gitea/services/mailer/sender"
@@ -95,6 +96,13 @@ func prepareMailerBase64Test(t *testing.T) (doer *user_model.User, repo *repo_mo
 	return user, repo, issue, att1, att2
 }
 
+func prepareMailTemplates(name, subjectTmpl, bodyTmpl string) {
+	loadedTemplates.Store(&templates.MailTemplates{
+		SubjectTemplates: texttmpl.Must(texttmpl.New(name).Parse(subjectTmpl)),
+		BodyTemplates:    template.Must(template.New(name).Parse(bodyTmpl)),
+	})
+}
+
 func TestComposeIssueComment(t *testing.T) {
 	doer, _, issue, comment := prepareMailerTest(t)
 
@@ -107,8 +115,7 @@ func TestComposeIssueComment(t *testing.T) {
 	setting.IncomingEmail.Enabled = true
 	defer func() { setting.IncomingEmail.Enabled = false }()
 
-	subjectTemplates = texttmpl.Must(texttmpl.New("issue/comment").Parse(subjectTpl))
-	bodyTemplates = template.Must(template.New("issue/comment").Parse(bodyTpl))
+	prepareMailTemplates("issue/comment", subjectTpl, bodyTpl)
 
 	recipients := []*user_model.User{{Name: "Test", Email: "test@gitea.com"}, {Name: "Test2", Email: "test2@gitea.com"}}
 	msgs, err := composeIssueCommentMessages(t.Context(), &mailComment{
@@ -153,8 +160,7 @@ func TestComposeIssueComment(t *testing.T) {
 func TestMailMentionsComment(t *testing.T) {
 	doer, _, issue, comment := prepareMailerTest(t)
 	comment.Poster = doer
-	subjectTemplates = texttmpl.Must(texttmpl.New("issue/comment").Parse(subjectTpl))
-	bodyTemplates = template.Must(template.New("issue/comment").Parse(bodyTpl))
+	prepareMailTemplates("issue/comment", subjectTpl, bodyTpl)
 	mails := 0
 
 	defer test.MockVariableValue(&SendAsync, func(msgs ...*sender_service.Message) {
@@ -169,9 +175,7 @@ func TestMailMentionsComment(t *testing.T) {
 func TestComposeIssueMessage(t *testing.T) {
 	doer, _, issue, _ := prepareMailerTest(t)
 
-	subjectTemplates = texttmpl.Must(texttmpl.New("issue/new").Parse(subjectTpl))
-	bodyTemplates = template.Must(template.New("issue/new").Parse(bodyTpl))
-
+	prepareMailTemplates("issue/new", subjectTpl, bodyTpl)
 	recipients := []*user_model.User{{Name: "Test", Email: "test@gitea.com"}, {Name: "Test2", Email: "test2@gitea.com"}}
 	msgs, err := composeIssueCommentMessages(t.Context(), &mailComment{
 		Issue: issue, Doer: doer, ActionType: activities_model.ActionCreateIssue,
@@ -200,15 +204,14 @@ func TestTemplateSelection(t *testing.T) {
 	doer, repo, issue, comment := prepareMailerTest(t)
 	recipients := []*user_model.User{{Name: "Test", Email: "test@gitea.com"}}
 
-	subjectTemplates = texttmpl.Must(texttmpl.New("issue/default").Parse("issue/default/subject"))
-	texttmpl.Must(subjectTemplates.New("issue/new").Parse("issue/new/subject"))
-	texttmpl.Must(subjectTemplates.New("pull/comment").Parse("pull/comment/subject"))
-	texttmpl.Must(subjectTemplates.New("issue/close").Parse("")) // Must default to fallback subject
+	prepareMailTemplates("issue/default", "issue/default/subject", "issue/default/body")
 
-	bodyTemplates = template.Must(template.New("issue/default").Parse("issue/default/body"))
-	template.Must(bodyTemplates.New("issue/new").Parse("issue/new/body"))
-	template.Must(bodyTemplates.New("pull/comment").Parse("pull/comment/body"))
-	template.Must(bodyTemplates.New("issue/close").Parse("issue/close/body"))
+	texttmpl.Must(LoadedTemplates().SubjectTemplates.New("issue/new").Parse("issue/new/subject"))
+	texttmpl.Must(LoadedTemplates().SubjectTemplates.New("pull/comment").Parse("pull/comment/subject"))
+	texttmpl.Must(LoadedTemplates().SubjectTemplates.New("issue/close").Parse("")) // Must default to a fallback subject
+	template.Must(LoadedTemplates().BodyTemplates.New("issue/new").Parse("issue/new/body"))
+	template.Must(LoadedTemplates().BodyTemplates.New("pull/comment").Parse("pull/comment/body"))
+	template.Must(LoadedTemplates().BodyTemplates.New("issue/close").Parse("issue/close/body"))
 
 	expect := func(t *testing.T, msg *sender_service.Message, expSubject, expBody string) {
 		subject := msg.ToMessage().GetGenHeader("Subject")
@@ -253,9 +256,7 @@ func TestTemplateServices(t *testing.T) {
 	expect := func(t *testing.T, issue *issues_model.Issue, comment *issues_model.Comment, doer *user_model.User,
 		actionType activities_model.ActionType, fromMention bool, tplSubject, tplBody, expSubject, expBody string,
 	) {
-		subjectTemplates = texttmpl.Must(texttmpl.New("issue/default").Parse(tplSubject))
-		bodyTemplates = template.Must(template.New("issue/default").Parse(tplBody))
-
+		prepareMailTemplates("issue/default", tplSubject, tplBody)
 		recipients := []*user_model.User{{Name: "Test", Email: "test@gitea.com"}}
 		msg := testComposeIssueCommentMessage(t, &mailComment{
 			Issue: issue, Doer: doer, ActionType: actionType,
@@ -512,8 +513,7 @@ func TestEmbedBase64Images(t *testing.T) {
 	att2ImgBase64 := fmt.Sprintf(`<img src="%s"/>`, att2Base64)
 
 	t.Run("ComposeMessage", func(t *testing.T) {
-		subjectTemplates = texttmpl.Must(texttmpl.New("issue/new").Parse(subjectTpl))
-		bodyTemplates = template.Must(template.New("issue/new").Parse(bodyTpl))
+		prepareMailTemplates("issue/new", subjectTpl, bodyTpl)
 
 		issue.Content = fmt.Sprintf(`MSG-BEFORE <image src="attachments/%s"> MSG-AFTER`, att1.UUID)
 		require.NoError(t, issues_model.UpdateIssueCols(t.Context(), issue, "content"))
diff --git a/services/mailer/mail_user.go b/services/mailer/mail_user.go
index 5a200a5fa7..68df81f6a3 100644
--- a/services/mailer/mail_user.go
+++ b/services/mailer/mail_user.go
@@ -39,7 +39,7 @@ func sendUserMail(language string, u *user_model.User, tpl templates.TplName, co
 
 	var content bytes.Buffer
 
-	if err := bodyTemplates.ExecuteTemplate(&content, string(tpl), data); err != nil {
+	if err := LoadedTemplates().BodyTemplates.ExecuteTemplate(&content, string(tpl), data); err != nil {
 		log.Error("Template: %v", err)
 		return
 	}
@@ -90,7 +90,7 @@ func SendActivateEmailMail(u *user_model.User, email string) {
 
 	var content bytes.Buffer
 
-	if err := bodyTemplates.ExecuteTemplate(&content, string(mailAuthActivateEmail), data); err != nil {
+	if err := LoadedTemplates().BodyTemplates.ExecuteTemplate(&content, string(mailAuthActivateEmail), data); err != nil {
 		log.Error("Template: %v", err)
 		return
 	}
@@ -118,7 +118,7 @@ func SendRegisterNotifyMail(u *user_model.User) {
 
 	var content bytes.Buffer
 
-	if err := bodyTemplates.ExecuteTemplate(&content, string(mailAuthRegisterNotify), data); err != nil {
+	if err := LoadedTemplates().BodyTemplates.ExecuteTemplate(&content, string(mailAuthRegisterNotify), data); err != nil {
 		log.Error("Template: %v", err)
 		return
 	}
@@ -149,7 +149,7 @@ func SendCollaboratorMail(u, doer *user_model.User, repo *repo_model.Repository)
 
 	var content bytes.Buffer
 
-	if err := bodyTemplates.ExecuteTemplate(&content, string(mailNotifyCollaborator), data); err != nil {
+	if err := LoadedTemplates().BodyTemplates.ExecuteTemplate(&content, string(mailNotifyCollaborator), data); err != nil {
 		log.Error("Template: %v", err)
 		return
 	}
diff --git a/services/mailer/mailer.go b/services/mailer/mailer.go
index bcd4facca9..db00aac4f1 100644
--- a/services/mailer/mailer.go
+++ b/services/mailer/mailer.go
@@ -43,7 +43,7 @@ func NewContext(ctx context.Context) {
 		sender = &sender_service.SMTPSender{}
 	}
 
-	subjectTemplates, bodyTemplates = templates.Mailer(ctx)
+	templates.LoadMailTemplates(ctx, &loadedTemplates)
 
 	mailQueue = queue.CreateSimpleQueue(graceful.GetManager().ShutdownContext(), "mail", func(items ...*sender_service.Message) []*sender_service.Message {
 		for _, msg := range items {
diff --git a/templates/devtest/devtest-header.tmpl b/templates/devtest/devtest-header.tmpl
index ee08545640..0775dccc2d 100644
--- a/templates/devtest/devtest-header.tmpl
+++ b/templates/devtest/devtest-header.tmpl
@@ -1,2 +1,3 @@
 {{template "base/head" ctx.RootData}}
 <link rel="stylesheet" href="{{AssetUrlPrefix}}/css/devtest.css?v={{AssetVersion}}">
+{{template "base/alert" .}}
diff --git a/templates/devtest/mail-preview.tmpl b/templates/devtest/mail-preview.tmpl
new file mode 100644
index 0000000000..9a3d792904
--- /dev/null
+++ b/templates/devtest/mail-preview.tmpl
@@ -0,0 +1,27 @@
+{{template "devtest/devtest-header"}}
+<div class="page-content devtest ui container">
+	<div class="flex-text-block tw-flex-wrap">
+		{{range $templateName := .MailTemplateNames}}
+			<a class="ui button" href="?tmpl={{$templateName}}">{{$templateName}}</a>
+		{{else}}
+			<p>Mailer service is not enabled or no template is found</p>
+		{{end}}
+	</div>
+
+	{{if .RenderMailTemplateName}}
+	<div class="tw-my-2">
+		<div>Preview of: {{.RenderMailTemplateName}}</div>
+		<div>Subject: {{.RenderMailSubject}}</div>
+		<iframe src="{{AppSubUrl}}/devtest/mail-preview/{{.RenderMailTemplateName}}" class="mail-preview-body"></iframe>
+		<style>
+			.mail-preview-body {
+				border: 1px solid #ccc;
+				width: 100%;
+				height: 400px;
+				overflow: auto;
+			}
+		</style>
+	</div>
+	{{end}}
+</div>
+{{template "devtest/devtest-footer"}}
diff --git a/templates/mail/auth/activate.mock.yml b/templates/mail/auth/activate.mock.yml
new file mode 100644
index 0000000000..f5519a6f6c
--- /dev/null
+++ b/templates/mail/auth/activate.mock.yml
@@ -0,0 +1,3 @@
+DisplayName: User Display Name
+Code: The-Activation-Code
+ActiveCodeLives: 24h