This best practices blog suggests deleting (or at least 'retiring') leads that hard bounce even once.
But what if people change jobs and you can have a sales rep look up their new email address?
Basically, do hard bounces ever "come back from the dead" - or should you really delete leads that hard bounce once?
"Marketo recommends deleting/retiring leads that hard bounce even once: Managing hard bounces: Retire all hard bounces after the first bounce. Most email providers and marketing automation solutions do this for you, but not all do, so make sure that any email that hard bounces is removed from your list. And if you’re using an ESP where you load email lists into the campaign from an external data source like SQL tables or Microsoft Access, be sure to regularly export all of your hard bounces and add them into a suppression list after each campaign. Then, scrub them against your email database when running a list selection."