That is interesting. Been playing with Exchange Online Mailbox types.
With the -Type parameter specifies the mailbox type for the mailbox. Valid values are:
- Regular
- Room
- Equipment
- Shared
Now I had never noticed this before, but when you switch a mailbox to anything other than Regular, the UserAccountControl changes to AccountDisabled.
That would explain why authenticated SMTP relay to smtp.office365.com would stop working :-o