Split email hosting using google and exchange

We have 500 mailboxes configured fora domain xyz.com on the in house exchange. We wish to add 200 new mailboxes on the same domain xyz. Com for external vendors , but on google apps. Eventually if everything works fine we would like to move all users to google apps. Currently how can we achieve the split hosting for a domain xyz. Com on both in house exchange and google apps
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Ilya RubinshteynCommented:
There are 2 ways to do this. One is to create a subdomain for the vendor accounts, i.e. abc.xyz.com. Public DNS servers concentrate on domain names and are not able to route e-mail properly based on individual address. Alternatively, you can set up a smart host for yourself that will route the e-mails based on the address/group. Set the smarthost as the primary mail server, and then configure appropriate routing rules within it.

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