If you use A2Billing on your SysAdminMan VPS then you may want to get it to send e-mail (for password reminders, invoices …). It is possible to use the local postfix mailserver and get your VPS to send mail directly. This can sometimes result in mail being marked as spam though.
A much better option is to relay your outbound e-mail via gmail. To do this you need a valid gmail account.
Here are the steps to take.
Go to SYSTEM SETTINGS / Global List / Global –
Enter the following settings, using you gmail account details –
You should be able to do this with your own e-mail server too if you have one in place.
These settings were taken from here – http://www.asterisk2billing.org/cgi-bin/trac.cgi/wiki/FAQ_gmail