Using remote host with postfix installed for my vbulletin forum to send mails

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http://serverfault.com – I m having a vbulletin forum hosted on a linux server where i also installed postfix. Locally everything is workin fine if i'm disabling smtp from the vbulletin email options but i wish to use SMTP now. I don't want to use google because i may get more than the smth google mail limit per day so i want to use my secondary linux vps where i also installed postfix. I've tried to add the second vps ip into smtp field and port 25 but did not worked. I tried to change postfix of the second vps config and instead of localhost i added the 2nd vps ip then tried again but still not working. How can i (HowTos)