Fetch / relay mails with External MX hosted ISP's server
I've zimbra 8.0.3 in my local domain (within my company premises), domain's mx pointed with my hosted domain provider's IP.
All users were access their mails direct domain provider's mail server before I implemented Zimbra, now I created same users on my local Zimbra server and I would like to fetch / sync mailboxes with all accounts and also relay the mail using their smtp server.
I don't want to point MX records directly on my local Zimbra server b'coz i've no any DR, spamming or availability resources. only would like to sync with external to internal server and relay mails from internal to external using providers SMTP server.
How can I set the same on my local Zimbra server ?