How to send e-mail from Ubuntu server?

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http://askubuntu.com – What I'm supposed to do is send emails from Ubuntu. The easiest way I have found is using Thunderbird and linking my real e-mail address ([email protected]) to Thunderbird account. I can successfully receive e-mails and download full history this way. I can't send e-mails, however. What I want to do is send e-mails straight from Ubuntu and appear as user1@localhost or something similar. Is that even possible? After reading different topics about this, It seems that using a relay address is the only way. I have also installed Postfix, same way as written there: How to set up a mail server? Whe (HowTos)