SMTP - Outgoing address(es)?

Is there a way to find out what IP source addresses might be used for outgoing mail? Also, are these stable enough to use in an IP filter?

Here’s my situation. I forward my incoming mail to my own mail server (via a subdomain with it’s own MX records). I’d like to restrict incoming SMTP to Dreamhost’s IPs since that’s the only place mail should be coming from.

I’m on the spunky cluster and I see that I get connections from I’m aware that there is an mx2. Are there other addresses - perhaps ones that are used for failover/emergencies?

BTW - I know there is an SPF source at (use “dig txt”). Is it kept up to date? Perhaps I could use that…