Sending Mail with Postfix

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http://www.unix.com – im able to install postfix on my ubuntu server. no issues. but ican't send mail. i dont know what to put for the "relayhost". i've googled this endlessly but i'm clueless. isn't there a general smart host i can use that'll allow me to send? i dont care for receiving mail. i just want to be able to send mail using the the unix "mail" command. any ideas? (HowTos)