The title is a little confusing. We have an internal mail server that can be used to send mail. This internal mail server is only accessible from within our firewall unless we use the mail server's portal.
I have a number of users who prefer to use Gmail due to the interface. While we can use Gmail to fetch mail from our mail server we can not use Gmail to send email due to the firewall (you can't send email such that your Gmail address does not appear in the header).
Basically, I want to set up a SMPT server that I will point gmail to. This server will reside within the firewall and it should be able to relay to our internal mail server. Does anyone know how I could do this? Also what would be some of the security implications I should consider?