This incident has been resolved.

All issues around sending email are now resolved and emails are sending again. Users should NOT have to refresh their email connections, but in case there are further error messages the email connection in question will have to be refreshed/reconnected. We are now monitoring the situation to make sure everything is back to normal.

Our prior quick fixes have not entirely solved the issues we're seeing with connected Microsoft accounts. We're now investigating details and reproducing issues in a controlled environment so we can make a more robust fix. We recommend holding back with sending emails through Microsoft accounts until these issues have been fixed.

We have identified an additional issue where users with connected Microsoft accounts/emails are not able to send emails even after refreshing their connected account. We are working on a fix.

We have applied a fix to the error handling so users should now see a better error message when sending email fails. Users will still have to reconnect their Microsoft account if there are errors.