Postbox setup issues?

Anybody have any issues setting up email in Postbox while using Avast? No matter which email address I enter I get this error:

"You are about to override how postbox identifies this site. Legitimate sites will not ask you to do this." and then "Certificate is not trusted."

Have quadruple checked to make sure all fields are entered correctly. Anything I’m missing? Thanks.

Export the avast certificate and import it into Postbox.