How to Create a Hub Transport Rule Based on Any Attachment

Tuesday, August 13, 2019
Transport rules are cool. Except when they don't work the way you expect.

I wanted to create a transport rule that blocks outgoing email to external recipients that contains an attachment, except for members of the "Allow Outbound Attachments" group. So, I created the following rule:

The trouble is, that it blocks any email that is not sent as plain-text, even though there is no attachment. Not good.

I examined the headers on emails with and without attachments and found the X-MS-Has-Attach X-header has a yes value when there's an attachment and is <blank> when there isn't.
...Message-ID: <>References: <>In-Reply-To: <>Accept-Language: en-USContent-Language: en-USX-MS-Has-Attach: yesX-MS-TNEF-Correlator:Authentication-Results-Original: spf=none (sender IP is )...
I modified the rule as below and it now works perfectly.

It's worth noting that the header values are not cAsE sensitive.

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