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m_c_7's avatar
m_c_7
Copper Contributor
May 18, 2020

How to decommission on-premises exchange servers and move completely to O365

We are currently using Exchange server 2013 in a hybrid setup with centralised mailflow through our on-premises servers.
All our mailboxes are in O365.
Following outbound connectors exist in O365:-
Office365 to Mimecast Journaling
Name RecipientDomains SmartHosts
---- ---------------- ----------
Office365 to Mimecast Journaling {journal.ourdomain.com.au} Mimecast Journaling Servers
Outbound to OnPremisesServerGUID {*} {OnPremisesServer.ourdomain.com.au}

Following send connectors exist on the on-premises exchange server with transport role:-
Identity AddressSpaces Enabled
-------- ------------- -------
Exchange On-Premise to Mimecast {SMTP:*;50} True
Outbound to Office 365 {smtp:OurO365TenantName.mail.onmicrosoft.com;1} True

We want to decommission the on-premises servers or may just retain 1 server for allowing relay from on-premises applications.
What will be the next step to decommission the on-premises servers?

I am thinking of following steps:-
1. Set up a connector in O365 to Mimemcast with "*" as the recipient domains and disabling the "Outbound to OnPremisesServerGUID" connector.
And then running the Hybrid configuration wizard to remove the centralised mail flow setting.
This should make the email flow from O365 directly to mimecast instead of going through on premises server.
I need to ensure that O365 spf is included in our spf.

2. Find out which applications are relaying through our on-premises exchange server and then make then relay through O365 using option 1 or 3 described in following article:-
https://docs.microsoft.com/en-us/exchange/mail-flow-best-practices/how-to-set-up-a-multifunction-device-or-application-to-send-email-using-office-3

3. Change Mimecast to route mail to OurO365TenantName.mail.onmicrosoft.com rather then our on-premises servers.

Is that plan correct and what testing should be done at each stage?

office 365
    • PeterRising's avatar
      PeterRising
      MVP
      May 18, 2020

      m_c_7 

       

      The only way that you will remain supported when removing your final on-premises Exchange Server is by breaking the synchronisation between AD and Azure AD, and adopting a cloud only identity.  You will be missing out on the benefits of Seamless Sign Sign on if you do that however.

       

      As already stated, leaving at least one Exchange Management server in place when your mailboxes are in Exchange Online, but the source of authority is on-premises AD is the supported position.  Gives you greater control of those on-premises attributes, and also allows you to use that Exchange Server for SMTP relay for your multi-function devices and any software which needs to send emails.

      • m_c_7's avatar
        m_c_7
        Copper Contributor
        May 21, 2020

        PeterRising NunoAriasSilva 

        Thanks Peter and Nuno.

        Based on this I am thinking of following revised plan:-

        1. Set up a connector in O365 to Mimemcast with "*" as the recipient domains and disabling the "Outbound to OnPremisesServerGUID" connector.
        And then running the Hybrid configuration wizard to remove the centralised mail flow setting.
        This should make the email flow from O365 directly to mimecast instead of going through on premises server.
        I need to ensure that O365 spf is included in our spf (for all our domains).

        2. Leave current exchange server that relays mail on-premises. Does that only need the CAS role (exchange 2013)?

        3. Change Mimecast to route mail to OurO365TenantName.mail.onmicrosoft.com rather then our on-premises servers.
        4. Remove following connector from on-premises server to make it route mail through O365:
        Identity AddressSpaces Enabled
        -------- ------------- -------
        Exchange On-Premise to Mimecast {SMTP:*;50} True

        Do you see any problems with this plan.

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