Verify your server settings. https://support.mozilla.org/en-US/kb/problems-sending-and-receiving-messages

Thanks for your reply. I did try to do that before posting. The option documentation from both gmail and thunderbird don't seem to match the current configurations but to the extent possible, I have matched them, particularly with respect to port number and authentication. There is something wrong that I may not be seeing or that is not fully documented.

There must have been a need for developers to be able to see more detail than the popup that indicates failed to connect.

Is there logging information that might help to understand why the failure is occurring?

Also, does the fact that I can send email(smtp) but not receive it, give a hint as to the problem with imap?

thank you

Can you post your Troubleshooting Information? Help (Alt-H) - Troubleshooting Information

Wrt logging, check this article. http://kb.mozillazine.org/Session_logging_for_mail/news

After several hours of work, I fixed the problem. Not sure exactly what cleared it but working now.

Also got messages about too many simultaneous connections. I think you may need to logout of gmail.

Saw a question about changing from pop to imap: File | New | Mail Acct Setup. This used the Mozzilla ISP database to establish the correct settings. That selected imap.google.com and port 993. That worked.

Also, I previously had toggled on to off for imap in google acct settings. Logged Off; Logged on: Toggled imap on; Logged Off.

So these muttering were what seemed to work.

The logging was not helpful but perhaps http://kb.mozillazine.org/Network_tools_for_server_connections would have gotten me the information faster that what I did or following christ1 suggestion to verify server settings. I suspected it was a server/port issue so the net tools such as using telnet to see the responses from the server might have been more helpul.

When your problem has been fixed can you mark the thread as 'Solved' please? Thank you.