Use auto-discovery for SMTP authentication method.

Replaced the hardcoded SMTPAuthPlain with SMTPAuthAutoDiscover to dynamically determine the best authentication method. This enhances compatibility with various email servers and reduces the need for manual configuration.

api/sendform.go

@@ -152,7 +152,7 @@ func GetMailClient(f *form.Form) (*mail.Client, error) {
 	}
 	mc, err := mail.NewClient(f.Server.Host, mail.WithPort(f.Server.Port),
 		mail.WithUsername(f.Server.Username), mail.WithPassword(f.Server.Password),
-		mail.WithSMTPAuth(mail.SMTPAuthPlain), mail.WithTimeout(serverTimeout))
+		mail.WithSMTPAuth(mail.SMTPAuthAutoDiscover), mail.WithTimeout(serverTimeout))
 	if err != nil {
 		return mc, err
 	}