You can use SaaS
The secure mail "pull" delivery method is indeed hosted on-box. However....
I saw a presentation at FOCUS12 that stressed the availability of the inbound SaaS cloud service and that it's often included in many license arrangements to relieve local MEG's from having to handle the full firehose of spam volume.
http://www.mcafee.com/us/products/saas-email-protection-and-continuity.aspx appears to be the page on it and contrasts the inbound only service with an apparently more comprehensive version of it.
If that matrix is to believed ""Email Encryption (Push/Pull Encryption, Pre-Built DLP Rules, Document Registration, Attachment Content Scanning (300+ File Types))" is a differentiator of McAfee SaaS Email Protection & Continuity versus the "McAfee SaaS Inbound Filtering." But the realities of that marketing sheet... I can't speak to.
I think rangerlj may be referring to that offering. Looks like it could possibly do what you want.