MFNs are not sent to remote domains

Symptoms 

Consider the following scenario: 

1. A partner organization uses Exchange Server on-premises or Exchange Online but is not a part of your Exchange organization.

2. A remote domain is configured to enable Meeting Forward Notifications (MFN) for the domain that's used by the partner organization.

3. You forward a meeting to any (internal or external) address that was scheduled by the partner organization.

In this scenario, no MFN is created to deliver to the meeting organizer. 

Resolution

1. To fix this issue, install the following cumulative update: 
   
   Cumulative Update 15 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019​​​​​​​​​​​​​​

2. Enable the fix by running the following setting override:
   
   New-SettingOverride -Name "MFNAllowSendToExternalOrganizer" -Component MailboxTransport -Section "MFNAllowSendToExternalOrganizer" -Parameters @("Enabled=True") -Reason "Enable MFN to be sent to external organizers"
   ​​​​​​​
   ​​​​​​​Get-ExchangeDiagnosticInfo -Process Microsoft.Exchange.Directory.TopologyService -Component VariantConfiguration -Argument Refresh

Reference 

• To configure and manage connection for a remote domain., refer Set-RemoteDomain.