Cannot get email from NAS...firewall settings?

I have a NAS that allows for errors to be emailed and I decided to use
GMail. However, it doesn't seem to be sending the emails. I suspect
that my router's firewall is blocking the port. I would like to know
what is the most secure way to get the emails out. I guess I need to
open the port listed below ONLY for outgoing and ONLY for (an IP, a
MAC addr?) but how do I do this?

Here are the settings and where I found them:
port: 465 or 587