diff --git a/docs/releases.md b/docs/releases.md
index b8ff0ca4..44b42243 100644
--- a/docs/releases.md
+++ b/docs/releases.md
@@ -1798,4 +1798,8 @@ and the [ntfy Android app](https://github.com/binwiederhier/ntfy-android/release
 
 ## Not released yet
 
-Nothing.
\ No newline at end of file
+### ntfy server v2.20.x (UNRELEASED)
+
+**Bug fixes + maintenance:**
+
+* Reject invalid e-mail addresses (e.g. multiple comma-separated recipients) with HTTP 400
diff --git a/server/errors.go b/server/errors.go
index 7a120e5b..77caf239 100644
--- a/server/errors.go
+++ b/server/errors.go
@@ -142,6 +142,7 @@ var (
 	errHTTPBadRequestTemplateFileNotFound            = &errHTTP{40047, http.StatusBadRequest, "invalid request: template file not found", "https://ntfy.sh/docs/publish/#message-templating", nil}
 	errHTTPBadRequestTemplateFileInvalid             = &errHTTP{40048, http.StatusBadRequest, "invalid request: template file invalid", "https://ntfy.sh/docs/publish/#message-templating", nil}
 	errHTTPBadRequestSequenceIDInvalid               = &errHTTP{40049, http.StatusBadRequest, "invalid request: sequence ID invalid", "https://ntfy.sh/docs/publish/#updating-deleting-notifications", nil}
+	errHTTPBadRequestEmailAddressInvalid             = &errHTTP{40050, http.StatusBadRequest, "invalid request: invalid e-mail address", "https://ntfy.sh/docs/publish/#e-mail-notifications", nil}
 	errHTTPNotFound                                  = &errHTTP{40401, http.StatusNotFound, "page not found", "", nil}
 	errHTTPUnauthorized                              = &errHTTP{40101, http.StatusUnauthorized, "unauthorized", "https://ntfy.sh/docs/publish/#authentication", nil}
 	errHTTPForbidden                                 = &errHTTP{40301, http.StatusForbidden, "forbidden", "https://ntfy.sh/docs/publish/#authentication", nil}
diff --git a/server/server.go b/server/server.go
index 075d3079..71f39357 100644
--- a/server/server.go
+++ b/server/server.go
@@ -122,6 +122,7 @@ var (
 	fileRegex                                            = regexp.MustCompile(`^/file/([-_A-Za-z0-9]{1,64})(?:\.[A-Za-z0-9]{1,16})?$`)
 	urlRegex                                             = regexp.MustCompile(`^https?://`)
 	phoneNumberRegex                                     = regexp.MustCompile(`^\+\d{1,100}$`)
+	emailAddressRegex                                    = regexp.MustCompile(`^[^\s,;]+@[^\s,;]+$`)
 
 	//go:embed site
 	webFs       embed.FS
@@ -1163,6 +1164,9 @@ func (s *Server) parsePublishParams(r *http.Request, m *model.Message) (cache bo
 		m.Icon = icon
 	}
 	email = readParam(r, "x-email", "x-e-mail", "email", "e-mail", "mail", "e")
+	if email != "" && !emailAddressRegex.MatchString(email) {
+		return false, false, "", "", "", false, "", errHTTPBadRequestEmailAddressInvalid
+	}
 	if s.smtpSender == nil && email != "" {
 		return false, false, "", "", "", false, "", errHTTPBadRequestEmailDisabled
 	}
diff --git a/server/server_test.go b/server/server_test.go
index 71743638..cb20cbda 100644
--- a/server/server_test.go
+++ b/server/server_test.go
@@ -1543,6 +1543,30 @@ func TestServer_PublishEmailNoMailer_Fail(t *testing.T) {
 	})
 }
 
+func TestServer_PublishEmailAddressInvalid(t *testing.T) {
+	forEachBackend(t, func(t *testing.T, databaseURL string) {
+		s := newTestServer(t, newTestConfig(t, databaseURL))
+		s.smtpSender = &testMailer{}
+		addresses := []string{
+			"test@example.com, other@example.com",
+			"invalidaddress",
+			"@nope",
+			"nope@",
+		}
+		for _, email := range addresses {
+			response := request(t, s, "PUT", "/mytopic", "fail", map[string]string{
+				"E-Mail": email,
+			})
+			require.Equal(t, 400, response.Code, "expected 400 for email: %s", email)
+		}
+		// Valid address should succeed
+		response := request(t, s, "PUT", "/mytopic", "success", map[string]string{
+			"E-Mail": "test@example.com",
+		})
+		require.Equal(t, 200, response.Code)
+	})
+}
+
 func TestServer_PublishAndExpungeTopicAfter16Hours(t *testing.T) {
 	forEachBackend(t, func(t *testing.T, databaseURL string) {
 		t.Parallel()