have you installed the proton mail bridge? without it you can not get proton mail in Thunderbird or any other mail client. Basically I fail to understand why anyone pays for their service. Especially given the number of folk posting here asking questions about proton mail.

For all I know that dialog could be for an outlook calendar or contacts and have nothing to do with email as such.

Thank you, Matt. I did install the bridge but the problem was because I was using the wrong password in the set up. It is working very well with Thunderbird now. The reason I like Proton Mail is there are no advertisements. I could no longer abide Outlook, not only because of the marketing but I would get locked out of my email several times a month with a message that they suspect suspicious activity. There was never any suspicious activity. Proton only costs $4 a month for privacy, security and no marketing. A good cup of coffee costs as much.

Thanks again. I appreciate your help. BP

Have a look at GMX, it costs nothing. Their web pages have adds, but not in a mail client and they do not track you like google and are not a US corporation so have to abide by the reasonable privacy requirements of the German government.

My choice really for an honest free offering.

Thanks for the suggestion. I will check it out.