Hello Allen,
I would suggest to share with us the configuration, I worked with this in the past. I would be interesting to see if you are receiving something in the logs.
Are you working with th OOTB receive email see link ?
[View:https://docs.appian.com/suite/help/20.4/Receive_Message_Event.html]
Even though that should not make a difference I have a quick question. Are you configuring using 'msg!properties'.'ToEmails'? 'msg!properties'.'ToNames’?
I remember that it is a little dependent of the sender how this was received. I would suggest to try using the 'msg!properties'.'To' and see what you get.
I hope this helps
Thanks
Jose