I still have the problem.
- the problem has only occured whit one particular external user (person1)
- no users can send an attachment to person1 from Outlook 2010. But everyone can log into office365 webmail and send the exact same mail and then person1 receive the attachment.
- If a user try to send a email from outlook 2007 to person1, the attachemnt is recieved by person1. Sp it has something to do with Outlook 2010.
Recreate users does not help