Christopher, I've had a similar error recently, but I solved it. Just to make sure if that is the case case, does it happen when you send single emails, too, or only when you send mass mailing? In that case, how many emails are you trying to send as once?
If you are sending mass mailings, before trying anything else, it is possible that your ISP is blocking outside e-mail servers so that you must use their outgoing e-mail servers. This is done by the ISP in an effort to block SPAM. MindSpring, AT&T and Videotron are known to be ISPs which do this. If your ISP does block outgoing e-mail servers you can use the Web Mail feature in your Cpanel to send/receive e-mail.
If you tried it all and you still get an error, then do this simple test. If you are using Windows, hit the START button and click on RUN. In the field, enter this:
telnet yourdomain.com 25
Observe the results. If you get your server information, then you can connect and problem is elsewhere.
However, if you cannot, then that means that your ISP has blocked it and is forcing you to send e-mails through their ISP's e-mail servers instead. That you cannot do anything about.
Btw, it would also be beneficial to see what the upstream provider limitations are in regard of mass mailing.
Best,