smtp is not only cryptic but also not wholly appropriate -- one never
gets information *from* smtp the protocol, and the act of simply
firing off a mail message via the system mailer doesn't really have
anything to do with smtp anyway.
mailto is the most appropriate one I can think of -- mail should be
reserved for reading mailboxes, for if and when that becomes an issue.
It makes little sense to mangle the address into something like
mailto://ncsa.uiuc.edu/marca; what happens with nonstandard addresses
such as firstname.lastname@example.org or email@example.com or
some of the more cryptic addresses (e.g., gatewayed through Sprintmail
-- anyone seen one of those yet? strange stuff...) that are starting
to pop up? Sure, there can be mappings established, but why bother?