I've been using James on my personal server since about 2.1.3. My domains receive a good deal of spam (on the order of ~1000 per day, spread over the e-mail accounts I host and the postmaster address).
Some of these messages contain attachments (usually virii) that seem to contain characters that will stop the POP3 transmission in the middle of a RETR.
Basically, this means that when you RETR an offending message, GUI mail clients time out and telnet sessions just hang until you escape to console and quit.
If the message is first in the list, this means most clients will not be able to retrieve the rest of the messages in the inbox.
The only end-user solution is to telnet in, DELE the offending message(s), and then RETR the good ones.
I'm going to try to attach a sample of an offending message (I haven't used JIRA much). Maybe this is a more general problem with some kind of control characters in the POP3 implementation?
I haven't had a chance to dig in to James's architecture, but I'd love some pointers.