Ability to send SMTP mail comes and goes

[bristophocles]

My ability to send SMTP mail comes and goes. I am not using SSL, squirrel mail works fine, and my config file seems okay. When messages fail, I get this line in my CPanel error log file:

PHP Notice: SMTP error: Connection failed: Failed to connect socket: Connection refused in /*mydomain*/roundcubemail/program/include/bugs.inc on line 95

I don't know why this error is returned, or why it will only happen for brief periods of time and then works just fine. Anyone experience similar problems or could possibly point me toward a potential solution? Thanks.