Exchange Online Mailbox Types

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