quota.x-username files appearing in /tmp
I've been using Ispconfig, running on Debian, using Postfix, Dovecot and Squirrelmail, for almost a year now. It has been working very well.
Recently, I noticed some quota files have been appearing in /tmp. They are in the format quota.mailx-username, where x is the unique number assigned by Ispconfig, and username is the user account created for a specific domain. For example, quota.mail29-joesoap:
-rw------- 1 mail29-joesoap web11 2387968 Sep 19 09:49 quota.mail29-joesoap
These files appear to contain mail output from the mailer daemon, which is addressed to the user in question.
My question is, why does mail that is apparently destined for a certain user end up in the quota file in /tmp? The size of the quota.mail29-joesoap file is usually several hundred kilobytes to a few megabytes in size, which obviously forms part of the user's own quota and exacerbates his quota over limit problem.
What causes these files to be created, and how do I solve this issue?