Investigate why mails are frozen

The exim documentation Show archive.org snapshot says:

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. This is probably the most common case, but there are also other conditions that cause freezing, and frozen messages are not always bounce messages.

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Read more Show archive.org snapshot

By default, frozen bounce messages will be deleted after 2 days and other messages after 7 days. This post on stackoverflow Show archive.org snapshot shows a good way to search for reasons why messages are frozen.

Delete frozen mails in the queue

Maybe you should check which messages are frozen and search for reasons before you delete them. You should ensure that the mails you will delete are just garbage and nothing important.

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