If you’re receiving errors along the lines of:
456 Address temporarily unavailable.
Or
maildrop: maildir over quota.
Or
status: deferred

It’s likely that the mail box in question has run out of storage space. In my experience this is usually due to the user not sorting through their emails and deleting old ones, and in one case there was hundreds of megabits in the .Trash folder that had simply not been emptied.

In order to increase the mail directory’s quota for the user in question, you need to run the following command from inside the users directory:
maildirmake -q 500000000S,1000000C ./Maildir

this will allow 500MB of Storage to be used, or 1000000 Correspondents (or emails) to be stored on their mail directory, whichever comes first.

After running this command you’ll need to restart the mta
/etc/init.d courier-mta restart

and make sure that the cache of problem email addresses is cleared:
courier clear all