I have been testing the effects of tightening up some of the postfix
options. In particular I set the hard error limit to 1 so that people
relaying via web proxys get chucked off as soon as they send any invalid
smtp command. However, I notice that this seems to cause a problem with
fetchmail when I reject an invalid address with a 554 response. It
seems that fetchmail doesn't read the response in this circumstance,
hence doesn't flush/bounce the message and everything backs up.
Loosening up the postfix options seems to allow me to download mail again.
Is this a known issue?