You may get an error "Berkeley DB error", the Berkeley DB is probably corrupt and also get information from exim_mainlog uisng tail command.

# tail -f /var/log/exim_mainlog


2015-01-11 12:54:28 Berkeley DB error: /var/spool/exim/db/callout page 3 is on free list with type 13

2015-01-11 12:54:28 Berkeley DB error: PANIC: Invalid argument

2015-01-11 12:54:28 Berkeley DB error: PANIC: fatal region error detected; run recovery

2015-01-11 12:54:28 Berkeley DB error: PANIC: fatal region error detected; run recovery

We can resolved the error once have removed the exim db directory. They can be found in /var/spool/exim/db

Stop the Exim  service
 
#  /etc/init.d/exim stop


Shutting down exim:                                        [  OK  ]

Shutting down spamd:                                       [  OK  ]

could just delete or move "db" directory
 
#  cd /var/spool/exim

#  mv db bak_db

Finally, start the Exim service, Exim recreate database directory and all you'll lose is the existing retry data.
 
# /etc/init.d/exim start


Starting exim:                                             [  OK  ]

0 processes (antirelayd) sent signal 9

/usr/local/cpanel/scripts/update_sa_rules: running in background