Do you read mail for the user on apok? My guess is that you have a crontab which is sending tons of mail to the user (which you'd notice if you used that user for mail), and since MAILCHECK is set in /etc/profile,
Setting MAILCHECK to a different interval should fix it, but won't... if this is the problem, you should do the following:
1) Clear out the mail for that user; if you don't receive any regular mail at your domain at that user, you can remove everything in ~/Maildir/cur/ and ~/Maildir/new/. Probably doing "rm Maildir/cur/*" won't work because there will be so many files - do:
for f in ~/Maildir/new/* ; do rm $f ; done
(same for ~/Maildir/cur/ if there's anything in there).
2) Prevent cron from sending mails by either sending stderr / stdout from your cron jobs to a file (or, more likely, to /dev/null), or by setting MAILTO="" in the crontab.
* * * * * /my/cron/job >/dev/null 2>&1
(sends stdout and stderr to /dev/null)
* * * * * /my/cron/job >/dev/null 2>$HOME/cron_errs
(I think should send stdout to /dev/null and stderr to a logfile)
Simply setting MAILTO="" will work for most people.