Format:Flowed not working in Mail 4.0?

When sending email in Mail 4.0, all of my outgoing messages are being line wrapped at ~ 70 characters per line. I would like for all of them to be flowed (e.g. no hard line breaks). All mail is being sent as plain text via a MS Exchange server.

In looking at the header of the outgoing mail, I see the following: Content-Type: text/plain; charset="us-ascii". It appears that Format:Flowed is missing. Did this change in Mail 4.0, or is this a function of the mail server?

Thanks!