I have had to update our google apps outbound from a certain provider (to stop using his services)However, I am not sure if that field should now stay blank? or is there any other value that I need to enter.I tried some help posts, but neither mention a default value for that fieldhttps://support.google.com/a/answer/178333?hl=en
Should there be a default value on the Outbound server in google Apps?
google apps;google apps email
Unless you are specifically using an alternate SMTP server for sending mail you should not enter anything into this field. If you are moving from one host to GAPPS for mail, you should only update your MX records. Here is the help article on how to do that.