July 4, 2019, 09:10 PT
Our e-mail team have implemented a fix for this issue, and outbound e-mails should no longer be rejected.
July 4, 2019, 07:55 PT
Users sending e-mail to certain external organizations may have their e-mail rejected by the recipient organization due to an issue with one of our mail relays. Our e-mail team are aware and are investigating. Re-sending rejected e-mails should result in them being successfully delivered.