Check high disk I/O commands:

1. iotop -oPa -d 2: High I/O on /var partition
2. glances: High iowait and load
3. iostat -2: /var high I/O confirmation
4. ls -lhS /var/log/*log | head: Large mail.log
5. tail -f /var/log/mail.log: Rapid, continuous logging

Freezing occurs when a bounce message encounters a permanent failure because the sender address of the original message that caused the bounce is invalid, so the bounce cannot be delivered. 

Command to view the queue

mailq

OR

exim -bp

View the body and/or header of a queued exim message

exim -Mvh 1XSY5g-0001kh-N6

 

The following command will remove all the frozen mails:

exim -bpr | grep frozen | awk {'print $3'} | xargs exim -Mrm

or

exiqgrep -z -i | xargs exim -Mrm