said by DownLow:... (and perhaps his encoding is what is messing up my text) ...
Unless your correspondent is using a Western European language based on the Latin alphabet, your "charset="us-ascii"" might be the problem with the text rendition.
If the "OrigIP" had been "64.98.156.XXX", it would have been possible to check for DNSBL listings.
Other than that, nothing in your headers suggests anything other than your guess that the problem is spam filtering on the receiving end.