Postfix and servers requiring authentication

[Jayson Smith]

Hi,
Today I got an Email from my ISP indicating that by next Tuesday I need to
tell my mail client to authenticate when connecting to my smtp server.  This
is aparently to help fight spam, which I understand.  However, I'm currently
sending mail to my local Linux box and having it relay it to my ISP's
server.  Is there a way to tell Postfix to authenticate with the remote
server when it relayes, and if so, how?
Thanks,
Jayson.