Thanks for your post in forum. Firstly, the voting feature requires the rtf (rich text format) message format. So if the recipients receive messages in other formats like html or plain text, they will not see the voting option.
So if the users send emails from the outlook client to other users, you can let them check in their outlook clients to see if they have set to send emails using rtf like the screenshot below.
If not, please let the users change the setting and then try again.
Note: For voting button
Microsoft Exchange account is required.