fix: Allow Auth over non-TLS SMTP connections (#6402)

* fix: Allow Auth over non-TLS SMTP connections

* remove unused struct

---------

Co-authored-by: Kitsune <kitsune@akitsune.dev>
Co-authored-by: Livio Spring <livio.a@gmail.com>

internal/notification/channels/smtp/plain_auth.go

@@ -0,0 +1,22 @@
+package smtp
+
+import (
+	"net/smtp"
+)
+
+type unencryptedAuth struct {
+	smtp.Auth
+}
+
+// PlainAuth returns an Auth that implements the PLAIN authentication
+// mechanism as defined in RFC 4616. The returned Auth uses the given
+// username and password to authenticate to host and act as identity.
+// Usually identity should be the empty string, to act as username.
+//
+// This reimplementation allows it to work over non-TLS connections
+
+func (a unencryptedAuth) Start(server *smtp.ServerInfo) (string, []byte, error) {
+	s := *server
+	s.TLS = true
+	return a.Auth.Start(&s)
+}