См. это полное учебное руководство для установки Openfire с решением типичных проблем.
It may appear that Hotmail in all its wisdom likes to check MX records in reverse priority order. It looks like our backup-backup-backup-backup mx record (priority 40) was confusing the matter. We'll see how this progresses...
UPDATE
Many years later and this is still working fine. Lesson learnt: Anything associated with M$ may well involve flawed logic.