If I see an email returned due to a hard bounce, after how many days is it acceptable to resend to that email address. It is possible for emails to be reactivated or for temporary outages, so it doesn't make sense to keep an email in my hard bounce email list forever. I've already seen cases where I receive emails from addresses that were put in my hard bounce email list months ago.
Any recommendations? Are there specific recommendations from ISPs?
There is no rule of thumb and that's why I have to go into greater detail:
Normally, I would believe what an email provider is telling me: A harbounce says that there are permanent problems with a certain email adress (i.e. it does not exist) while a softbounce says that there is only a temporary problem (i.e. exceeded disk quota by the user).
Believing the provider, I would drop email adresses with a hardbounce from my list while keeping the ones with softbounces. There could be several reasons for a softbounce which may need user interaction by the email owner (i.e. freeing some disk space on the server by deleting emails). So you cannot predict a point in time where the email could be delivered to the user. But you could try resending based on an upscaling sequence like the Fibonacci numbers and dropping the address when reaching a certain limit.
Not believing the provider, you could additionally count hardbounces for every address. This strategy takes problems on the mail provider side (sending false hardbounces) into consideration. I recently worked with a commercial newsletter system. They were using a counter for hardbounces and softbounces each. Since you normally do not send a newsletter every hour to the same address, there is enough time that technical problems on provider side could be fixed. The default limit for hardbounces was 3 and the newsletter was not resent after a hardbounce. An adress was only deactivated after reaching the global limit and was reset after delivering an email successfully.
