How to track down possible module conflict? I am trying to get the SMTP Authentication Support module working on my development site. Whenever I try to send a test message, I get the error "SMTP -> ERROR: Failed to connect to server: Permission denied (13)." Our SMTP server is configured to accept relay from any client on our internal network, which is where my server lives. When I look at the logs, I see two errors - one of type "mail" and one of type "smtp".  Each say about the same thing.  This leads me to believe that the SMTP module is conflicting with something, but no other modules are currently installed.  I have tried/removed other versions of the SMTP module and the PHPMailer module.  I can't find a "mail" module in the Core section, so I don't think that's it.

When I install the module on my local machine, in a clean Drupal instance, with all the same configuration settings, the test message flies right through.  So, I'm guessing that maybe one of the other modules left some cruft behind in the database somewhere, and this may be causing a conflict with the SMTP module.  However, I don't know how to hunt down and eradicate the problem.

Any ideas?
After much beating my head against the wall, I found a solution:

https://drupal.org/node/1593684

Turns out, it was SELinux snuffing out the SMTP attempt before it could even reach the server!  Doh!