Well, I’m still somewhat puzzled – but have a partial answer today. The client’s contact eventually called me to report that all emails (including the ones I requested) which were blocked had PDF attachments. I asked her to read one of the non-delivery reports to me over the phone, and there was a reference to the size of the attachment….so I will have to go with that. The contact protested that she often sends much larger PDFs, this was a single page which should have passed through without issue (she has previously sent 68 page PDF files without issue) but I’m guessing there was file corruption involved. I could not confirm this, as she was unable to give me a file size…..but that’s where it stands.
Newer PDFs may also have content other than embedded text. There can be executables, web links and other more active content. This could also be blocked by some mechanism along the path. It does seem strange that there is no other information in the NDR’s. Some email filters are not “smart” so try renaming the file attachment to something other than PDF.