Notes ttachments become embedded in body

The senders are using Lotus Notes in another domain. The attachments they send to us sporadically are becoming embedded into the body of the message when Exchange 5.5 sp4/Outlook 2000 client receives them. The attachments are still encoded and cannot be read. This happens regardless of the type of attachment (Word, Zip file, etc.). We are the only recipients of email from this domain that is having the problem.