How to compose the MimeMessage when using the twitter filter in Java?

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Can anyone show how to create a MimeMessage readable by Twitter?

The e-mail is being sent to the recipients fine, and applying the
twitter filter looks straight-forward, but the message doesn't show up as a Twitter udpate.

I believe the issue is how I'm formatting the message itself, not with how I apply the twitter filter to the X-SMTPAPI header.

Thank you.