I had this problem solved several years ago … but of course I had forgotten it. I hope this helps someone else out.
I have Comcast Internet which has blocked port 25 in sending email, and now allows port 50. That port can change any day now, and without warning, no thanks to spammers. I upgraded my MEPIS Linux to version 8 last night and it went without a hitch. I was configuring Thunderbird for email and I could receive email, but not send. I knew that I needed to add a TCP port of 50 to Guarddog as you can see in this picture.
The second part I had forgotten. You have to go back to the PROTOCOL tab of Guarddog and go into the “User Defined” section to enable that protocol. Click the “Apply” button and all of a sudden Thunderbird is as happy as a clam.
I hope this helps someone else out on the Intertubes. That someone might be me, after I’ve forgotten this in a couple of months.