It's a crypto signature used by Outlook and Mail: http://www.fileinfo.net/extension/p7s
Actually Outlook doesn't understand it. It's not unique to Apple Mail either (although I am using that). It's a from Thawte.
smime.p7s
Description: S/MIME cryptographic signature
/* PLUG: http://plug.org, #utah on irc.freenode.net Unsubscribe: http://plug.org/mailman/options/plug Don't fear the penguin. */
