For those who are suffering from following the instructions of how to use an external SMTP connection, yet continue to fail, check out this technote, which reveals that for those using Cpanel or WHM you need to change the following settings:
Login to WHM panel , Main >> Server Configuration >> Tweak Settings >>
Restrict outgoing SMTP to root, exim, and mailman (FKA SMTP Tweak) :
Make above function Set to : OFF
external smtp errors when using Cpanel or WHM
NOTE TO ADMINISTRATOR OF FORUM: What’s up with inputing optional tags? I input a few to help people find the correct area of concern, and they are never saved.