We are in the process of implementing Proofpoint. There isn't a Mac outlook plugin, but I expect that will change when the new Office goes out of preview and they have access to microsoft's plugin architecture. Proof point does some email scanning and is supposed to auto encrypt outgoing sensitive data in email as well as having subject triggers so the client plugin isn't required.
Since you mentioned email apps specifically I assumed you were looking in that sort of direction, DLP is pretty broad (data control, device control, etc...).
Still in the pilot phase so it remains to be seen how usable it will be in production.
I'd love to know if there's a good solution here. In a dream world, we'd be able to force user's to label mail messages & even other documents, and do some content inspection for email, docs, and writing to approved external drives.
McAfee and Symantec have both roadmapped DLP, but I haven't seen it working yet.