Your servers can direct mail through Google Apps using SMTP or SSL.
Using SMTP out port 25, you can send mail to any Gmail or Google Apps users
Using SSL out port 465 or 587, you can send mail to anyone.
For SSL capable devices, connect to smtp.gmail.com on port 465 or 587.
A Google username and password for authentication is required. Ensure CAPTCHA has been released and a very strong password is used for this account.
If your device or application does not support SSL, connect to aspmx.l.google.com on port 25
Be sure to add this IP to the Google Apps Email Whitelist as well as add it to the SPF record. See Email Whitelist and SPF Record