I've written an article before on how to change the outgoing SMTP port for SmartHost in Exchange 2003. Now, I have to do this again for Exchange 2007 as they've changed this procedure.

When you're unable to send e-mail directly to the internet (using MX), you probably have to set up a SmartHost to relay the e-mail for you. This is basically your ISP's SMTP server, but can be another server as well.
In certain cases, the default SMTP port (port 25) cannot be used to connect to the SmartHost, so you have to change this to something else. Gmail for instance, uses port 587 for SMTP. Other 3rd party mail services use this (or another port) as well to circumvent spam protection initiated by a lot  ...