fix: support SMTP STARTTLS mode and NTLM auth (#5426)

* fix: support SMTP STARTTLS mode and NTLM auth

Add explicit SMTP STARTTLS configuration for 587-style connections and keep SSL/TLS as the implicit TLS mode.

Prefer PLAIN when advertised, keep LOGIN compatibility, and add NTLM as a fallback for Exchange SMTP servers that require it after STARTTLS.

* fix: respect explicit SMTP encryption mode

* fix: preserve SMTP TLS compatibility

common/constants.go

@@ -120,6 +120,8 @@ var InsecureTLSConfig = &tls.Config{InsecureSkipVerify: true}
 var SMTPServer = ""
 var SMTPPort = 587
 var SMTPSSLEnabled = false
+var SMTPStartTLSEnabled = false
+var SMTPInsecureSkipVerify = false
 var SMTPForceAuthLogin = false
 var SMTPAccount = ""
 var SMTPFrom = ""