I have been investigating further and it certainly looks like a problem
with Courier IMAP, not Exim. Given that a number of people on this list
use these two togther, I hope no-one will mind me asking about it here.
I have modified my Exim configuration to append the message size to each
message filename as someone suggested could be helpful.
Do I need to do anything to tell Courier IMAP to enable quotas? I have
it configured to retrieve user details, including quota, from a
PostgreSQL database. The Courier documentation suggests that each
Maildir should have a file called maildirsize. I haven't got any of
these. What is supposed to be responsible for creating it, the IMAP
daemon? Exim? Me?