Exchange 2007 issue: PDF attachments with name longer than 20 characters won't go through

Odd issue here I've been intermittently dealing with for months now. Certain PDF's sent as email attachments inbound cannot go through. I have confirmed the issue is with the PDF attachment filename length. Other PDF's with shorter names go through just fine. From my testing, the max length the PDF can have is about 20 characters, even though I read that Exchange '07 should be able to process up to 78, at a minimum.

Also noteworthy is the fact that if the test PDF with a long name (about 60 characters) is sent internally on our domain, there are no problems and it goes through fine.

I've been searching online for hours now and I cannot find Exch Mgmt Shell commands or Exch Mgmt Console (GUI) instructions to even check the attachement character limits, much less change them. Any/all help appreciated. Thank you.