Outlook XP Attachment problem

I am a IT intern and a user has a strange problem with Outlook which I cannot seem to figure out.

The problem is that he cannot recieve any attachments from a certain email address (outside of the company), the email itself arrives but just without any trace of the attachment.
While if the exact same email is send to someone else in this company everything is just fine.

Seeing this I thought the problem must be on the users laptop/email account since he is basically the only one who can't recieve the emails with attachment within the company.
But after checking if he didn't had any spam filters or whatever ditching the attachment (even tho if that were the case the whole mail should have been seen as spam), looking if Outlook itself didn't remove the attachment, checking the blocked attachement extentions for Outlook in the regestry (even tho they should then still show up in the mail) and looking if the server didn't somehow deleted the attachment from the mail.

The company which the other user sends the emails from have checked there server settings etc. multible times (like we did here) and could not find anything.

I am clueless atm about how this is even possible, often it is a simple setting which I must have missed but after searching on Google it doesn't even seen to know this problem exists...

Is there anyone here who might have a suggestion about what could be not only blocking but completely removing the attachment for this 1 user?

I have actually never resolved this problem or found anyone who could.