We're ramping up an internal campaign against invalid sender and recipient domains. Below should serve as a more detailed explanation.
As a general rule, we cannot assume that a domain which does not presently resolve is an invalid domain. The domain could be experiencing a temporary DNS issue, or we could be experiencing one ourselves. For this reason, email that you send to or from an invalid domain sits in queue and we try again to deliver it every few minutes. This becomes problematic when users collectively send to and from hundreds, or thousands, of actually invalid domains every hour. It causes email queues to explode in size, and it makes it more difficult for us to actually identify problems on our platform as the statistics can be masked by invalid domains that will stay in queue for the maximum amount of time configured.
Automating the blocking of invalid sender/recipient domains has proven to not be a reliable job, as a temporary DNS issue does occasionally occur, and that would cause us to block a valid domain as invalid. Moving forward, we will be performing audits on our platform for invalid sender/recipient domains multiple times throughout each day. The goal is this is to quickly return to you a clear error message at SMTP time when you've attempted to send email to or from an invalid domain.
An example of an invalid domain and one that we just blocked prior to posting this: @pop-os.localdomain