John,
What other cloud solutions are you asking about?
I have gotten O365 to work with an internal SMTP prevent server that then talked to a Cloud Proofpoint MTA. This was done WITHOUT the use of the DLP for CLOUD license too!
Gmail is not ready for primetime yet..there is no configuration (that I know of) that allows routing of mail for the next hop in Gmail.
Good Luck
Ronak
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