Exchange 2K7SP1 flagging mail with subject personalisation as spam

I sent a campaign yesterday to a bunch of clients, and even though the campaign's spam test was green across the board, a subset of recipients - all at the same domain - got Soft Bounced as "spam detected".

The campaign had a subject in the form "Dear [firstname], blahblahblah." It wasn't until I removed all personalisation and changed the subject to just "Blahblahblah" that the campaign was accepted by the domain mailserver. I called the mail admin at the recipient domain and he informed me that they're running MS Exchange Server 2007 SP1.

Has anyone else encountered this sort of thing?

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