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