If you find some error in the exim_mainlog like the one shown below
--------------------
2008-09-06 06:39:05 H=techs.ahosting.biz [72.29.84.175] temporarily rejected connection in "connect" ACL: Host is ratelimited
2008-09-06 06:40:05 H=techs.ahosting.biz [72.29.84.175] temporarily rejected connection in "connect" ACL: Host is ratelimited
--------------------

The fix is here...type the command.

/usr/sbin/exim_tidydb -t 1h /var/spool/exim/ ratelimit