SMTP mailer issue

Within the SMTP mailer module settings with the 'Enable SMTP mailer' box unchecked I'm able to send a test email with the form at the bottom of the page with success. When I check the 'Enable SMTP mailer' box the mail sending fails. Is it just me? The server is clearly setup correctly if it's able to send a test email... but when 'enabled' it fails! Any ideas?

So I've realised when the 'Enable SMTP mailer' box is unchecked it is simply using the default SMTP settings and not the SMTP Module's content. I'm able to use the same username, password, hostname and port 587 from Thunderbird and Outlook with success... but not from our Dolphin site - any ideas?

NB: We are using an external mail server, it is not the same as our Dolphin route - any ideas?

If the outbound server your using in your SMTP settings is not your own server. In other words, external instead of internal, and you are sure the settings are 100% correct, then the only conclusion i can come up with is your host or your own server has a firewall or other security system in place that is blocking the outbound port.

 It is an external server as mentioned above and I'm sure the settings are 100%. Our server's firewall is not blocking ports 25 or 587... the mystery continues.

Still running a blank on this one. Our server has the aforementioned ports open, not blocked, not firewalled... and I'm able to use the exact same setting for outgoing mail using Thunderbird. Really odd.

So I had a dig around our WHM:

WHM-home »Server Configuration »Tweak Settings 
tab: Mail 
Restrict outgoing SMTP to root, exim, and mailman (FKA SMTP Tweak) 
 

This option was enabled so it's now sending the test emails from the SMTP module correctly... however, they are not actually arriving! How can I debug what's happening after the test mail is successfully communicating with our third-party mail server?