After a week of researching (off & on) I discovered that it's possible to use the "A Record" in a Postfix DNS query rather than the "MX record" if it exists, by enclosing the SMTP server name in [ ] (http://www.postfix.org/STANDAR…o%20MX%20record%20lookups. )
This became a problem for me when my router restarted and being allocated a blacklisted IP address from my ISP. When the MX record was used, the DNS resolving "outlook.com" was returning the IP of their 'protection' frontline servers that rejected connections because of this blacklisting.
I confirmed the solution through a 'smtplib' based Python script that had no problems delivering emails. The difference came to light when I sniffed the DNS query traffic (Wireshark) showing that 'smtplib' used the A Record and Postfix queried the MX Record.
More research indicated that Postfix accommodates this with the [ ] around the server name.
(It just happens to be a 50% solution for me as my 'production' Openmediavault is still on Ver 3.x and doesn't allow '[ ]' chars in the "SMTP Server" name field, whereas my test Openmediavault (ver 4.x) does. Another reason to upgrade!)
I hope someone finds this useful!