Introduction
You might come across the above issue when you setup a new system that should use your Exchange server as mail-relay. You may wonder how this (the error) could happen, invest several hours in research and funny or not, you'll find only Office 365 'solutions' - and some programming related.All in all - you might not find a solution that is the one for you.
A potential solution...
... might be as simple and stupid - and make you maybe banging your head against the wall, as I considered afterwards - just to cross-check whether the FROM-Email is the same Email-address - or in the pool of Email addresses, which have been configured for the account you are using to send your Emails.At least if all your attempt to solve the issue failed, it might be right the moment to double check what you - and I - should have checked first ;-)
