smarthost requires smtp authentication

[spartantechman]

I would like all outbound SMTP traffic to relay to a remote SMTP relay. The relay provider that I am using requires SMTP authentication. I've already added a smarthost via the smtproutes file in the /var/qmail/control directory with the following syntax ":example.relay.com". Mail is indeed relaying to my provider. However, because they require authentication, it is being denied. Where do I need to supply this authentication information in qmail? I'm using Plesk 7.5 on Fedora. Thank you in advance for your assistance.