Attachments are missing from some senders

[Chris Barber]

We have a new issue that cropped up about a week ago on our CentOS MailScanner installation, running the latest stable MailScanner release.  For some reason, when some senders send a PDF file as an attachment, the email comes through but there is no attachment. In Outlook, there is the little paperclip icon on the message indicating there is an attachment, but alas the attachment isn't there. There is also no notice from MailScanner that anything happened to it. 

So, I white listed the sender. However this did not change the behavior, still no attachment. If the same sender sends to a non-MailScanner protected domain, the attachment comes through fine. 

I'm at a loss really where to even start since there is no attachment warning from MailScanner. Where should I look first?

Thanks!
Chris