Preventing certain ssh tunnels

[Tom Limoncelli]

(I'm using openssh 2.5.1p1)

Is it possible to configure sshd so that incoming connections can only
make certain tunnels or restrict which hosts tunnels can be directed
at?  (Say, only to the imap4 server and smtp, or restrict tunnels from
going to a certain host)

I can't find anything like this in the sshd man page.  Maybe something
creative can be done with the use of entries in authorized_keys.

Thanks
--Tom