How do I mail from an Internet to a uucp site?

[Jeffrey W Percival]

I'm sitting here at my Ultrix Internet site and I get mail
from a!b!user at foo.edu...

How do I mail back to this user?  I have not found the right combo
of backslashes, percents, and so on to please my mailer.

Do I type something different depending on whether I'm "in mail"
or typing to the shell?
-- 
Jeff Percival (jwp at larry.sal.wisc.edu)