How can I stop undeliverable bounce messages (e.g. to routeable, but undeliverable, spammer senders) from clogging up the queue for days?

If at all possible, you should try to avoid getting into this situation in the first place, for example, by verifying recipients so that you do not accept undeliverable messages that lead to these bounces. You can, however, configure Exim to discard failing bounce messages early. Just set ignore_bounce_errors_after to specify a (short) time to keep them for.

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