Invalid Emails - Max time without delivery

Hi,

When handling invalid emails, we have an initial alert to handle any mis-spelled or 'does not exist' emails, where the company can then investigate the error and attempt to clean the database. However, after running this for some time we are now noticing that most of the invalid email reasons are 'exceeded max time without delivery' messages.

I'm looking for advice on how to handle these. I can't find documentation in resources to explain how Marketo handles them. If we were to run the next email campaign and exclude email invalid (as we do as a matter of course on best practice), then this marker will still remain and exclude these people. However surely they need to be unmarked invalid after 72 hours for a reattempt?

Can anyone offer any advice?

thanks

Sarah