If you wish to have exim send an warning to your email users when their inbox is reaching it's limit, edit your /etc/exim.conf, find the virtual_localdelivery:, just after the line that begins with quota = add:
quota_warn_threshold = 80%
quota_is_inclusive = false
quota_warn_message = "\
To: $local_part@$domain\n\
Subject: Your mailbox\n\n\
This message was automatically created\n\
by mail delivery software.\n\n\
The size of your mailbox $local_part has exceeded \n\
a warning threshold of 80 percent\n\
of your maximum mailbox size.\n"
Save, exit, and restart exim.