External Meeting Requests are failing to be forward to internal mailboxes.

I'm having a very odd issue ever since I've migrated our on-prem exchange to exchange online. It appears that any external meeting request cannot be forwarded to an internal email address. (Using the Outlook 2016 client) There's no error or kick back from the system. They appear to go into limbo. I've checked my mail flow rules and have no explicit rule disabling this function unless there was an internal default to block these forwards.
Any assistance would be appreciated.
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Office 365 Pro Plus clients
Spam filter used Proofpoint essentials.
Steven KutasovicSystems AdministratorAsked:
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Vasil Michev (MVP)Commented:
Define forwards though? Actual email forwarding from the end user via Outlook? Have you run a message trace on one of the forwarded messages?
Steven KutasovicSystems AdministratorAuthor Commented:
Hi Vasil,

Yes, the end user are forwarding the meeting request from their Outlook 2016 client.
After running a message trace. I discovered that these external meeting requests are being blocked from forwarding to internal contacts. Throwing this error Error: 550 5.2.1 Message deleted by the transport rules agent
Which is very strange to me because the only transport rules I have are a spam filter receive and send from proofpoint essentials. And a couple of disclaimers. Are their any hidden mail flow rules that would not appear on the admin center gui?
Steven KutasovicSystems AdministratorAuthor Commented:
It appears that it was my default mail flow policy and specifically the "Outside your organization" parameter. Since the external meeting requests originated from an external address the transport rule will only look for the FROM field and not the SENDER. Causing it to fail and thus no email would be generated for the end user. Find more in this thread.

Tech Net thread detailing fix.

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Steven KutasovicSystems AdministratorAuthor Commented:
Internal Mail Flow item was causing issues as the FROM field was not being rewritten to on-prem causing it to fail when sending external meeting requests.
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