Hi,
If these are external users sending attachments to users in your network, then there may be some restriction at their end. Email is not a good way to distribute large attachments, one of the commonist ways of minimizing the impact of this is for ISPS to block sending of large attachments.
If this is not the case then do the users get the messages, even though there are no attachments? If so, could this be an Outlook problem distinguishing the attachment from a specific mail configuration?
My point being that if you can receive mails without attachments, unless you have nstalled content checking software at your end, the MTA won't kare whether an attachment exists or not, and won't touch the attachment.
Andrew.