just a tip: are you sure the smtp.gmail.com accepts connection from everyone? If it does, why has it a non-default SMTP port then?
(also are you sure that your firewall has rule to allow outbound port of 465? )
If you check the MX records of gmail.com, neither mail server is named like smtp.gmail.com:
gmail.com MX preference = 20, mail exchanger = alt2.gmail-smtp-in.l.google.com
gmail.com MX preference = 10, mail exchanger = alt1.gmail-smtp-in.l.google.com
gmail.com MX preference = 40, mail exchanger = alt4.gmail-smtp-in.l.google.com
gmail.com MX preference = 30, mail exchanger = alt3.gmail-smtp-in.l.google.com
gmail.com MX preference = 5, mail exchanger = gmail-smtp-in.l.google.com
These should accept mail on port 25. Try one of these directly.
I'm sorry, I think I misunderstood your situation. I read about gmail usage from external mail client and what I think your problem could be is that ePO cannot use SSL or TLS durign SMTP authentication.
Thanks. I suspected that it was an EPO limitation. It will give me more ammunication for upper management to use an internal email server for server notiifcations.