qmail + gmail configuration

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http://serverfault.com – I have VPS with Plesk 11 and I'm using two domains on it. I would like to set up the email service in order to work with two mailservers at the same time: On gmail I have an account [email protected] . On my server on qmail I've configure other mailboxes like [email protected], [email protected] etc. Now, as long as I can understand, I can just keep one MX server that I guess should be the legacy one (qmail)... correct me if I'm wrong. In this case if I try to send an email, qmail will obviously bounce back replying: 550 sorry, no mailbox here by that name. (#5.7.17). How can I tell (HowTos)