Postfix reports this error because it is responsible for 'example.com'
and the message has 'Delivered-To: u...@example.com'.
There are several options:
1) Your error. Your system is sending mail out after adding the
'Delivered-To: u...@example.com' header, and it comes back from
outlook.com. If Postfix did not block such email then it could loop
forever.
2) User error. After your system adds the 'Delivered-To: u...@example.com'
header, some of your users forward their email off-site. That email
ends up at a system that looks at the message header address and
that forwards that email to outlook.com. This results in a similar
problem as (1). If Postfix did not block such email then it could
loop forever.
2) Outlook error. Outlook.com adds the 'Delivered-To: u...@example.com'
header. In that case all mail from Outlook.com would have this
problem. It does not seem likely.
My money is on (1) or (2).
Wietse
