Investigated and hopefully I have got further.

Here are the continuous Hack attempts at the server…

This is the connection attempt, from the smtp server log, by the GL-X750

Finally, the log from the GL-X750

My interpretation of this is that my email server cannot service calls with Encrypted passwords. My eMail clients all use plain Passwords and when I attempted to connect with an encrypted password, It didn’t even attempt to connect to the server, or at least, it didn’t get as far as being logged…

I guess now my question is @alzhao ,
As we can no longer choose the Protocol for SMTP, what method does the router use to send password to SMTP server, NORMAL or some form of ENCRYPTION?

I have looked in ssmtp.conf and both removed AuthMethod and also set AuthMethod=PLAIN (default AuthMethod=LOGIN).

Not sure where to go from here. A new server install is on the cards for the longer winter evenings, but, I think I will bring it forward.