Doug has three different Windows 10 computers, but programs operating differently on all three. Incredimail has gone away, and so he tried Mozilla Thunderbird. Leo says that it probably has to do with how each computer was set up, which prompted POP3 instead of IMAP. You'll need to uninstall Thunderbird and reinstall. You have to use an IMAP server and port. What Leo suggests exporting the profile on the right Thunderbird computer, and import it into the other two.