Prevent spoofing of internal domain

Is there anything internally on the Exchange side that I can do to prevent the spoofing of our internal mail domain.  We have been getting some emails appearing to come form internal users to other internal users.  Upon inspection of the suspicion email the mail from looks legitimate but the but the actually reply address and return path are external internet addresses.

Is there a setting on the receive connector I can use to prevent this?  Should annoymous but allowed or disallowed on the receive connectors for example?
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You want to be careful not to remove anonymous from all of your receive connectors lest you stop getting any outside mail at all. What you probably want to do is make sure you have set up an SPF record for your domain and have some type of anti-spam solution in place. This article right from this site should help.
Gareth GudgerSolution ArchitectCommented:
What are you using for message hygiene? A lot of appliances and cloud providers will reject any messages claiming to be from the internal domain but coming from an external source.

As Tshearon pointed out an SPF record can help as well.
georgedschneiderAuthor Commented:
Couldn't you use something similar to the following which will remove rights to the anonymous user on the the receive connector to prevent spoofing of the internal domain coming form the outside:

Get-ReceiveConnector “Inbound Email” | Get-ADPermission -user “NT AUTHORITY\Anonymous Logon” | where {$_.ExtendedRights -like “ms-exch-smtp-accept-authoritative-domain-sender”} | Remove-ADPermission
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georgedschneiderAuthor Commented:
To answer the previous question I'm using Exchange Online Protection for message hygiene currently.
Gareth GudgerSolution ArchitectCommented:
Get-ReceiveConnector “Inbound Email” | Get-ADPermission -user “NT AUTHORITY\Anonymous Logon” | where {$_.ExtendedRights -like “ms-exch-smtp-accept-authoritative-domain-sender”} | Remove-ADPermission

I have seen this documented a lot out there but never used it. The big question is why is Exchange Online Protection allowing the domain spoofing through. Never seen that on any of my EOP clients. Have you placed a ticket with EOP?

Also, make sure you are only accepting mail from EOPs IPs.

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georgedschneiderAuthor Commented:
Let me reach out to EOP to see why this got past.  I agree this shouldn't have happened.
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