HELP!! I install UBUNTU 7.04 as MailServer, the system let me receive mail, but don't let send external mail

(
i follow the steps from
The Perfect Setup - Ubuntu 6.06 LTS Server (Dapper Drake) - Page 5
so i don't know what to do.
I tried modify main.cf mynetworks including my local ip/24 but don't work.
I'm using outlook in my clients and when i check for "Server needs authentication" always ask for user and password, and i write it and ask me again and again and don't let me see if the mail has send.
(sorry my English its not good)
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