Emails are not current and password does not work

When I open Thunderbird (Mozilla), my files are not current. In other words, the most recent emails, probably since the last time I used Thunderbird, in my file folders are missing. I have tried changing the server settings but I think they are correct. Please be patient with me, I am unfamiliar with all the terminology, etc. Also, when I first enter Thunderbird, it does not recognize my password. A window opens with the message "Login to server imap.postoffice.net with username john.fn@usa.net failed." That message also appears when I click on Get Messages. Thanks for any help.