First of all, I am unaware as to how one might be able to set IRM (Information Rights Management) permissions using only a standard SMTP message (MailMessage). IRM permissions, I believe, are completely Outlook/Exchange-centric, and lack any sort of representation in the SMTP standard.
The only way you're going to be able to enable that permission is through the Outlook Object Model via the Outlook Interop assemblies. IamStalker above me recommended Redemption, which, while 99% of the time is the correct answer for everything Outlook-related, is not the way to go for this particular problem. Because IRM (as far as I know) has no representation in MAPI, Redemption wouldn't be much help here; I don't believe I've ever seen an IRM-related property on Redemption's object model.
You'll most likely be interested in the Permission property exposed by the MailItem object type. It appears that in order to programmatically turn on the "Do Not Forward" option, you will want to set the value of that property to OlPermission.olDoNotForward.
There is another IRM-related property (PermissionTemplateGuid), but it appears that you can leave this as an empty string since you are using a built-in "permission".
I would assume then that all other IRM-related settings are provided to Outlook through the user's Outlook profile.
Good luck!