Somebody on the team exports bounces from the ESP once a month and deletes them from the CRM. Then the CRM syncs back and the dead addresses return.

It is fixable, since most sending platforms can post bounce events to a webhook, and a small script can move those addresses into a suppression list that every system reads from.

uChecker wrote up automated bounce processing with webhooks, logs and scripts, including what to do when the ESP gives you no webhook at all.