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QUESTION
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I want to setup a few of our email users on Office 365 or Hosted Exchange, but want to leave the bulk of our users still hosted on our regular IMAP/POP/SMTP server. Is this do-able?

 

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ANSWER
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Yes – simply follow these steps, and you will be all set:

Setup the regular SMTP/POP/IMAP Mail Server:
1. Leave MX records pointing to your regular SMTP/POP/IMAP email server.
2. For the users migrating to Hosted Exchange or Office 365, leave their accounts active on the SMTP/POP/IMAP mail server, but set them to forward to <user>@<domain>.onmicrosoft.com

Setup the Hosted Exchange or Office 365 email accounts

Setup Mail Flow on the Office 365 or Hosted Exchange accounts:
1. Exchange Admin Center
2. Service Settings, Mail section, click Anti-malware policies
3. Mail Flow
4. Accepted Domains
5. Select the domain from the list and click the ‘pencil’ icon to to Edit the domain
6.┬áSelect ‘Internal Relay’ from “This accepted domain is:” list
7. Click Save button
This removes the ‘Authoritative’ setting, thereby allowing external MX record lookups when emailing to internal users to allow the mail to flow outside the Hosted Exchange or Office 365 organization and to the regular POP/SMTP/IMAP server.

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EXAMPLES
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APPLIES TO / KEY WORDS
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Hybrid
Split Domain

 

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REF
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http://www.anysitesupport.com/office-365-hosted-exchange-hybrid-domain-domain-coexistence/
http://anySiteHosting.com