It does make a difference. IMAP is server based and it shows you the messages when it receives them. The settings you are playing with are for POP.

Thanks again, Airmail, for your reply. The odd thing is that my laptop, which runs Win 7 and uses the same server, will only check for messages when I tell it to do so. I installed Thunderbird on it only a couple of months earlier than I did this one. Is there some way to fix this?

Is Thunderbird set for IMAP on both computers for this account? The email account can be IMAP but you can set the client as POP or IMAP.

Also go into server setting for this account and click Server Setting and then Advanced. Uncheck Use Idle Command and see if that helps.

Thanks much, Airmail! I think you're two for two here. I unchecked and OK'd the Use Idle Command, and then I was able to make Thunderbird look for messages. I also checked my laptop and, sure enough, it listed a POP server, not an IMAP server. How does one set the server? Is it by the port number? Is one better to use than the other? My server's POP3 port number is 110, while its IMAP port number is 143.

Sherry

This article describes switching account types. https://support.mozilla.org/en-US/kb/switch-pop-imap-account

Thanks again, Airmail!