[Exim] User authentication to enable relaying

(How) is it possible to authenticate a "user" that may be connecting
from different networks onto a non-open relay.

For example: a person (ie me) wants to take his portable from place to
place, connect to the network I happen to on and send mail via my
"normal" SMTP relay which is currently configured to relay only from
certain IP addresses?

Is this a job for TLS/SSL certificates?

Dirk
--
Please Note: Some Quantum Physics Theories Suggest That When the
Consumer Is Not Directly Observing This Product, It May Cease to
Exist or Will Exist Only in a Vague and Undetermined State.