Russell Coker wrote:
There are ways of altering the binary message which shouldn't change the result when it's decoded. One example is base64 vs mime encoding, one could theoretically write a mail server that converted one to the other with most users not noticing a difference.
AFAIK base64 *is* a MIME CTE. By "mime encoding" are you referring to quoted-printable? https://en.wikipedia.org/wiki/MIME#Content-Transfer-Encoding
Messages with DKIM signatures can be checked.
Since he only cares about the attachment itself, it might be simplest to "sha512sum foo.doc" on each end. If he gets the same sum on each end for a doc that "doesn't work", the problem isn't en route.
Historically some relatively popular MTAs altered messages. [details]
Likewise; I've seen plenty of mail bits (especially BTSs) do silly things to messages, but corrupting the contents of an attachment is unusual. Changing the MIME type to application/octet-stream is more common, but that won't change the behaviour if you copy the attachment to disk before you open it.