Send a warning email to email account owners when they approach their inbox quota

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.