Funny error that took half a day trying to debug, the mail server always responded with “no such user here <blu@blux.com>” when I was trying to send the email to the blux guy (somewhere out on the internet) from my personalgolfclub.com domain with a completely valid account and credentials.
Found this post: https://www.nopcommerce.com/boards/topic/40148/error-sending-e-mail-mailbox-unavailable-the-server-response-was-anotherdomaincom-no-such-user-here
So created a new user and used his credentials and everything works nicely …. sounds a bit like the old user’s password on the smtp gateway was somehow corrupt or the pw not up to date, anyways … devlife goes on 🙂