Forum Discussion
How to decommission on-premises exchange servers and move completely to O365
Hi curious7,
If you remove your exchange server you are in an unsupported configuration, please read more here https://docs.microsoft.com/en-us/exchange/decommission-on-premises-exchange?redirectedfrom=MSDN
Best Regards,
Nuno Árias Silva
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.
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
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Exchange On-Premise to Mimecast {SMTP:*;50} TrueDo you see any problems with this plan.
