Is port 25 the only port I can use? Is there a way I can verify port 25 is open?
Yes... Using the embedded mail server in the 3300... No encryption, no authentication, no TLS, no SSL, nothing but old school plain text SMTP port 25.
You can test it with telnet from your laptop on the same subnet as the phone system... "telnet mail.server.com 25" and enter the commands manually.
https://www.simplescripts.de/smtp-check-port-25-telnet-command.htm is one reference, there are lots that Google can find for you.
This is kind of an outdated way of sending emails, especially with today's hosted solutions. The usual way of doing this now is using the MiCollab server as an email server, and pointing all email from the 3300 to the MiCollab server, or any MSL server like a MBG if you have one.