We are planning for migration from Microsoft Exchange 2013 to Microsoft 365, so our on-promises Exchange is giving us problems of having to restart services more often. Therefore I am asking whether is advisable to use "Hybrid Migration" or "Online Only"? and Will it fix the DMARC issue?

Well those two things arent related, but if you route all your outbound mail through Office 365, you can leverage DKIM and with your valid SPF record any messages sent from Exchange Online user mailboxes will pass DMARC https://video2.skills-academy.com/en-us/microsoft-365/security/office-365-security/use-dmarc-to-validate-email?view=o365-worldwide If you choose Hybrid or if all your mailboxes are in Exchange Online, it doesn't matter. As long as all the outbound mail routes through Office 365 then you are good.

Hybrid Migration? - Microsoft Q&A

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Andy David - MVP 144.4K Reputation points MVP

2020-10-08T14:23:15.25+00:00

Well those two things arent related, but if you route all your outbound mail through Office 365, you can leverage DKIM and with your valid SPF record any messages sent from Exchange Online user mailboxes will pass DMARC

https://video2.skills-academy.com/en-us/microsoft-365/security/office-365-security/use-dmarc-to-validate-email?view=o365-worldwide

If you choose Hybrid or if all your mailboxes are in Exchange Online, it doesn't matter. As long as all the outbound mail routes through Office 365 then you are good.
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Mr Godiraone G. Lefatshe 21 Reputation points

2020-10-08T14:33:26.783+00:00

Thank you for your response Andy.So the services of the on-premises exchange (2013) wont interrupt the online exchange (365)? I restart the mail server and the services every morning, so it is hideous job.

Andy David - MVP 144.4K Reputation points MVP

2020-10-08T14:36:46.057+00:00

Depends on what service :)

If you are in hybrid and have mailboxes in Exchange Online and on-prem, then mailbox moves and free/busy lookups between Exchange Online and on-prem could be affected.

If the server is in such bad shape that you are worried about it, I would bring another server to be used as the hybrid endpoint or move the mailboxes as quickly as possible to Exchange Online.

Mr Godiraone G. Lefatshe 21 Reputation points

2020-10-09T08:23:00.383+00:00

Following on your reply i have noted that the server might be in bad shape or there is other external issues. Our on-prem exchange is running on Hyper V (virtual machine). Sometimes all of the on-prem exchange service can run well and be a able to send mails outside the organization but then we struggle to receive external mails(or they take a longer time to get in). Please what can i use to troubleshoot this problem?

Kael Yao-MSFT 37,666 Reputation points Microsoft Vendor

2020-10-20T06:43:26.007+00:00

@Mr Godiraone G. Lefatshe
Hi,
It may be resulted from some backpressure problems.
Please check the application log to see if there are some relevant warnings or error messages.

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