We’ve been making some changes on our back-end mail servers that resulted in a number of accounts being moved to a new server.
This was done without disruption except for those who are SMTP forwarding to Microsoft 365.
This disruption was caused by Microsoft having some anti-spam ‘greylisting’ technology that caused them to greylist this new server because of the volume of emails coming from it.
To rectify this problem, we have moved all the affected accounts to another server that has been sending volumes of email to Microsoft for years, so isn’t affected by this problem.
So, many thanks Microsoft, and many apologies to all those affected by this.