You should immediately remove any 5xx "hard bounces"
I do, but, what would you do with something like
failed,5.0.0 (undefined status),"smtp;521 5.2.1 ""TempFail""
This address (an aol.com address) was regularly receiving emails without a problem until this one pops up. The address itself is valid. Our service is pretty 'clean' (own IP, DKIM, domain keys, spf, we react to fbls within a few days, and our content is newsletters and similar material from schools to parents)
This is a typical example of 5xx bounces that happen a lot where the 'English' - ie "undefined status" and "TempFail" do not suggest a need to remove them from our db.