Could someone help answer a few questions regarding the networking / DNS requirements for an SMTP server? Assume the server is setup already.
Is a MX record required for SMTP? TCP port 25 open to the server? Any other DNS records?
You may want to consider an SPF record as well.
You need port 25 open. An MX record is preferred, but any RFC-compliant server will fall back to the A record for your domain if one doesn't exist.
I noticed you used the relay tag - that functionality may be disabled by default, depending on what you're using. You'd need to add the upstream host to the relay host's list of trusted SMTP sources.
Outbound mail servers don't need an MX record but be advised that some email providers (most notably AOL) will check for a reverse DNS (PTR) record for the FQDN your server provides when communicating. I'd suggest adding a PTR.