This could happen if:

• Outbound port 25 is blocked either by your firewall (unlikely) or by your ISP. Are you using Xeams behind a static or dynamic IP. Note that if you have a dynamic IP address issued by your ISP, it is very likely they have blocked port 25. Most ISPs do not allow running an email server if you get a dynamic IP from them.
• Your IP address is blocked by the receiving end, which could happen if Google gets many junk messages from it.

Try opening a telnet session to Google's SMTP server. Use the following command:

telnet gmail-smtp-in.l.google.com 25

If you see an SMTP banner, connection is established. If not, something is blocking the connection.