From time to time I still use mutt to check or delete mail. For instance where I only have terminal access to a root account and something caused the account to collect thousands of e-mail. Here is how to delete a lot of mutt e-mail.

In the main screen press D (shift + d)
Delete messages matching: ~s .*

~s will search the subject for the given expression and ".*" is a global regex wild card

All messages will now be marked for deletion. Press q and y to confirm.