Microsoft → Outlook Mail Being Partially Blocked - Spam Protection?

A little help diagnosing what appears to me as overzealous corporate anti-spam interfering with emails.

For the past two days, a client of mine (European) is only partially receiving emails from me (usually blocking attachments of any kind and including some gibberish where my text is) - with a header that reads as below (I hid some information with Xs):

Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: quoted-printable
X-OriginatorOrg: XXX.net
Return-Path: XXX@XXX.net
X-MS-Exchange-Organization-AuthSource: chef-mailfront.XXX.mail
X-MS-Exchange-Organization-AuthAs: Anonymous
X-MS-Exchange-Organization-PRD: XXX.net
X-MS-Exchange-Organization-SenderIdResult: Pass
X-MS-Exchange-Organization-SCL: 0
X-MS-Exchange-Organization-PCL: 2
X-MS-Exchange-Organization-Antispam-Report:
DV:3.3.5705.600;SID:SenderIDStatus Pass;OrigIP:XXX.XX.163.240
MIME-Version: 1.0

I have instructed my client to contact his IT people to inquire, but it seems pretty obvious to me that his AntiSpam is interfering with my email (and perhaps his encoding is what is messing up my text). He is my only client having this issue as all other emails I send are normal.

Thanks for any insights you can provide.

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.

thanks for this. Interesting comment about the language - he is Dutch and I often see emails from him non-translated.