Voting button not visible to external exchange users in the email
Note: For the voting option to work email should go out as Rich text format.
Resolutions: To solve this issue, follow the below steps.
Create a new remote domain for the partner
POWERSHELL
New-RemoteDomain -Name partner domain -DomainName
partnerDomain.com
Once the remote domain is setup Open remote domain properties, Select "Message Format" under the "Exchange Rich-text format:" select "Always Use" and save the settings.
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POWERSHELL