Postfix working with home network but not with USB 3G dongle

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http://askubuntu.com – Does postfix with php's mail command get configured during installation based on the network connection? I am asking this because I installed it when I was connected to my home network. and I can send emails no problem. However, when I need to work remotely, and connect to the internet using a USB 3G dongle, I can no longer send out emails from php. When I get back home, and connect to my home network, I can send emails via php perfectly again. (HowTos)