Sometimes people get hacked, either because they pick a abc123 password (nowdays blocked by policy), or because a customer webserver CMS theme has not been updated in the last 2 years. When that happen, a few mail servers (mostly yahoo) has temp banned mails for a few hours. Mails still got delivered however.
So lesson to learn there is to not use passwords like abc123, and to actually update that 2 year old CMS theme. Or you don't combine webserver with webmail.