The problem was because either due to a miscommunication a long time ago with someone at Tucows, or to my misinterpreting the documentation for the Email Defense product (likely the latter), I had set the delivery mailbox (where they get their mail) for customers using both email defense and Tucows email (as well as for some using just the email product, but considering EDS) to "outbound.domain.tld.emailmx.com". This worked for many months but stopped working night before last when Tucows changed some internal structuring of how email is handled, and "outbound..." no longer responded to POP3 queries.
Thanks to Yura for helping to figure this out.
-tj
