Quite often Exchange hosting clients need to host part of mailboxes (for test or for VIP persons) on Exchange hosting, while another part of mailboxes on some third-party mail server (cheap mail hosting, or in-house mail server or even free mail service). Here are steps to make it work.

• MX record for client.com can be pointed to Hosted Exchange system or third-party mail server, the following scenario will work in both cases.

• Mailboxes on third-party mail server have only addresses like user1@client.com, user2@client.com. Mailboxes on Hosted Exchange has addresses like user3@client.com, user4@client.com (set us reply/primary addressees) and additional addresses like user3@client.hosterdomain.com, user4@client.hosterdomain.com. Usually hosterdomain.com set as temporary domain in DNP and domain client.hosterdomain.com is already automatically bounded to client's organization.

• MX record for client.hosterdomain.com pointed to Hosted Exchange system. Mailbox for user3@client.com created on third-party mail server, but set to redirect all mail to user3@client.hosterdomain.com.

• Set client.com as Internal Relay Domain and create send connector pointing to third-party mail server on Exchange according to "Internal Relay Domain" section in this http://technet.microsoft.com/en-us/library/bb124423.aspx article.