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 below 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.

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NOTE for SpamBlocker 4.x+:

If your "virtual_user" director uses LMTP (transport = dovecot_lmtp_udp), then the virtual_localdelivery will never get called, so the above guide won't apply.
When using LMTP (all new SpamBlocker versions), any E-Mail quota notifications would need to be done through dovecot's quota warning system:
http://wiki2.dovecot.org/Quota/Configuration#Quota_warnings

A guide has been created here:
http://help.directadmin.com/item.php?id=625