Email Continuity With O365 and On Prem Exchange

Hello. I am hoping someone can provide some pointers for a solution I am trying to figure out.

I have a hybrid configuration with O 365 and on prem Exchange. We also use Mimecast gateway.

So, normal email flow will be through Mimecast > O365 > On Prem Exchange.

This is what I am trying to figure out. If Mimecast has an outage is there a way for me to "easily" take Mimecast out of the flow. So that mail now flows through O365 > On Prem Exchange?

Can I add another MX record for O365 and set it to a lower priority? And add alternate send and receipt connectors from O365 to on prem Exchange?
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Mohammed KhawajaManager - Infrastructure:  Information TechnologyCommented:
There is more likely to be an outage in your environment or even Office 365 than an outage on Mimecast.  Having said that, you could have a secondary MX record pointing to Office 365 and ensure that is configured to accept email for your domain versus accepting connections from Mimecast.

I personally would use Mimecast if journaling is required and besides that, I would use features in Office 365 (i.e. anti-spam, anti-virus, archiving, etc.).

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Jian An LimSolutions ArchitectCommented:
I am actively working with Mimecast so probably some answer can be answer.

mimecast, office 365 MX are proper MTAs. this means they observe resend when services are not available.
one of things you can do is lower your MX record TTL from 3600 to 900. this will give you 15 minutes to swap them over.

For example, Mimecast offers grey listing, i.e. if a non-register MTA (probably spam virus) attempt to send the email, Mimecast will offer to reject the emails and wait for resend.  This feature might not available in Office 365.
IT also have a better UI then office 365. for example, you know how many emails Mimecast have reject, with whatever reason. with MIcrosoft you got minimal report you can show to.

TTL is your best bet. I won't put office 365 MX in your current MX as spammer knows which one is a weaker link :)
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