Active mode is pretty weird. Coordinating a single client across two ports sounds difficult to me, but I've never implemented it. If that's not a difficult protocol, then what is?
Cordinating a client across two ports sounds trivial compared to for example properly implementing client and server versions of IMAP search commands when no client or server follows the specification.
You will have similar issues with other protocols as well. I have seen physical bandwidth limiters, windows drivers, corrupt winsock LSPs etc mess with IMAP traffic. I have seen middleware replacing SMTP commands sent from a client to server for no good reason, such as replacing "EHLO hostname" with "EHLO **".
