Yeah, interesting...for a mailing list there are at least three possibilities - mailto: URI, info page, URI for direct posting to list over HTTP.
The latter two should be able to describe themselves to any agent that goes looking - @type is redundant (though useful). I suppose with mailto: the URI scheme plays a similar role to the media type with HTTP URIs. I don't know of any neat machine-friendly way of saying "don't dereference this with http" beyond parsing out the scheme. But maybe that's not a problem - if the mailto: is displayed as a link in a browser then the browser will typically do what's expected; any machine-oriented agent which might want to go dereferencing should have the smarts not to go chasing mailto:
s.