I would like to relay a specific Exchange 2010 Mailbox to an external SMTP host, in this case Office 365, as we're migrating user by user to Office365. Meaning if the migrated users receives an e-mail, it should be automatically redirected to the defined host (Office365).

In Postfix this is really simply done by using the transport map /etc/postfix/transportmap:

firstname.lastname@domain.com       smtp:[domain.mail.protection.outlook.com]

Any ideas how this could be implemented using Exchange 2010? Exchange Transport Rules don't have any action available for this scenario.

Thank you in advance for your help!

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    Any particular reason you're not using the hybrid deployment method built in to Exchange? Why are you re-inventing the wheel? – longneck Sep 12 '16 at 14:36

In Exchange you need to create a mail enabled smtp contact, then after in the account mailflow you forward to that external's contact.

How to configure mail forwarding Note The forwarding address can be any mail-enabled object. Examples of mail-enabled objects include distribution groups, security groups, public folders, and other related items.

  • Start the Active Directory Users and Computers snap-in.
  • Right-click the mail-enabled user, and then click Properties.
  • Click the Exchange General tab.
  • Click Delivery Options.

    • In the Forwarding Address section, click Forward to, and then click Modify.
    • Click the mail-enabled user or the mail-enabled contact. Then, click OK.

    Note If you want e-mail to be delivered to the original mailbox as well as the forwarding address, select the Deliver messages to both forwarding address and mailbox check box. To close Delivery Options and to close Properties, click OK two times.

Took from there How to configure a mailbox to forward mail to a mail-enabled contact

  • Hey yagmoth555, thanks for your input but I need an option to redirect to another smtp server and not just another smtp account. – joelschmid Sep 13 '16 at 5:51
  • @desperado it work for another smtp server :) if you create the contact aa an external account (like a hotmail.com or yahoo.com account) – yagmoth555 Sep 13 '16 at 10:26
  • I'm not sure if I understand you correctly. You actually just setup a mail forwarding on the desired mailbox? If I define a external forward address on a mailbox, does it keep the complete mail header? – joelschmid Sep 13 '16 at 17:13
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    @desperado Yes, you define a contact in exchange, and you select that contact to have the email forwarded to that contact. I do that for some user with iPhone with gmail, to forward their email to such device. When they answer as it's in their gmail's folder, it appear as coming from gmail, but the email is intact. On a normal outlook you could change the send from and the remote user would see nothing about that forward. In fact I even did this with some blackberry with their associated email, as you can change the mail from, and it was invisible. – yagmoth555 Sep 13 '16 at 17:21
  • Perfect! I just figured out that using Office365 I can even skip the contact creation as I can use the @tenant.onmicrosoft.com address to forward to. Thanks for your help! – joelschmid Sep 14 '16 at 6:42

According to Microsoft Support there is no way available to redirect a specific mailbox to another smtp server. However there might be some third-party software available to solve this problem. Just google for ''Sender based routing on Exchange''

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