Our email client will no longer send email via a relay service that has worked fine for the past 5 years or more.
We had some issues with our email provider in that they changed the mx records for our domain before being asked to do so, i.e. before the alternative was fully setup and configured, so for a brief time the domain would not allow receiving or sending of mail even though the outgoing has always used a relay service, so I don't understand why this would be so.
They've now been reset and mail is now coming in again but we can't send. I've created an alternative smtp relay service but we still get the same error message email-send-error.png
I'd check that with the recipient - maybe there's some subtle typo, or that guy no longer works there and his mail accound got disabled (bad idea if customers mails are not redirected to another account).