Site Administration, Server, Support Contact, Support Email = ABcdef@Test.com
Above is the actual From address used by Moodle. So this address must be the same as the authentication account details in the mail settings:
Site Administration, Plugins, Messaging Outputs, Email...
SMTP Hosts: smtp.office365.com:587
SMTP Security: TLS
SMTP Username: ABcdef@Test.com
SMTP Password: office365password
SMTP Session Limit: 1
No-reply address: ABcdef@Test.com
This is what clinched it for me.