Well, with Comcast you never really know what is going on part of the time, and if you are having sporadic problems even with them, you may be having (hopefully temporary) connectivity issues. .
There is another wiki page that I think you should work through:
That is a pretty good guide, and takes you step-by-stpe, so you might be able to find a “missed step” by reviewing that page. Note you will need to use port 587. Note that “mail.mydomain.tld” means “mail”+".+“yourdomainname”+"."+“Your Top Level Domain” (.com, .net, .org, etc) and not actually “tld” . I suspect you know that, but from reading your reply I just wanted to clarify that.
Also, make sure you do check the “My Server Requires Authentication” box on the “Servers Tab” (last picture) and that you DO NOT have the “Log On Using Secure Password Authentication” box checked, in either the “wizard” or the “Server Tab” of “Properties” (same screen as the one previously mentioned.)
If you follow those steps, and watch out for those potential “gotchas”, you should be up and running soon.