there's several things you can do here. Funny thing is that I just switched my roundcube to use SMTP instead of PHP. I was getting similar errors as you have displayed. I had to enable SMTP_DEBUGGING to get more information (defaults.inc.php). It helped me a bit, at least it showed me the connection was working, but the authentication was indeed failing. One thing you will see is how it's trying to authenticate (login, plain, etc) . What I found is that with my connection needed to use PLAIN in order to work. I had it set up on a the TLS port (587) so I think, and I'm no expert on this, that it should be fine. I'm also using "sendmail" (not sure what you're using). If you're still having issues, I might be able to assist. No more authentication issues. I also used: testsaslauthd to test my sasl authentication.