But three over 6 months would imply they're being blacklisted and/or their mail sender is simply broken.
Plus some enterprises change their mail policy out of nowhere. I have a lot of subscribers from enterprise companies, but sometimes every mail delivered to some company comes back with something like "Your address is not on our whitelist. We only accept approved emails." and every subscriber is hard bounced off the list and that's that. (They aren't paying me, though, so less of a big deal.)
Plus some enterprises change their mail policy out of nowhere. I have a lot of subscribers from enterprise companies, but sometimes every mail delivered to some company comes back with something like "Your address is not on our whitelist. We only accept approved emails." and every subscriber is hard bounced off the list and that's that. (They aren't paying me, though, so less of a big deal.)