I thought I'd post this becuase there is some bad information out there. Some guides tell you to edit /etc/postfix/master.cf (-o smtp_bind_address=) but this doesn't work. The same guide also says if you don't change it there you end up changing the listening IP/bind interface which is also not true.
Here is a simple and effective way to change Postfix's sending/binding/outgoing IP address (very important for reverse DNS and so mail servers don't block you):
vi /etc/postfix/main.cf
#add this option
smtp_bind_address=192.168.5.80
*Obviously change the 192.168.5.80 to the outgoing address you want to use for Postfix
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