Update: After two days' trying just about every suggestion in this forum to no avail, my e-mail started downloading again, as if nothing had been wrong. Everything was left exactly as it had been before this started. -- No changes made to the system. I would like to continue pursuing a solution to the original problem, because I'm sure this will happen again.

The only out-of-the-ordinary thing I did just before it began to work again was to try printing a document. The job remained in the print queue, showing a status of "Printing" but not 'printing. After trying to cancel the job, the status changed to "Deleting", but still would not clear. I stopped the spooler, tried (unsuccessfully) to delete the job, and restarted the spooler. To try to clear everything, I rebooted. This is when Thunderbird started working again.

I mention this only in case someone can see some relationship between the two issues. I doubt there is, but I'm sure there are many of you who know more about this than I do.

I see two possible relationships.

1. Anti virus these program do simply hang and usually take the communications stack with them. A reboot would fix the hang

2. Anti virus again, but this time the firewall component just looses the plot and blocks everything.

My experience is if you have a weird problem or something really unusual the best first step is turn the device off, unplug it and go have coffee before starting it again (that ensures the capacitors have a chance to discharge it only takes a minute usually, but any excuse for coffee)

Second step it restart in the operating systems safe mode with or without networking, depending on if you need to contact the internet to test the issue.

In the case of mail programs ensure that the anti virus is set to exclude wherever mail is stored from on access scans. Make things much faster and affects security about zero.

In the case of Norton's anti virus, they do not scan mail on secure connections (SSL/TLS) which excludes all the free mail providers I am aware of and most ISPs. Their mail scanning is about obsolete but no less intrusive than ever.

Thanks, Matt, for your reply. All of it makes sense, and I had tried everything you suggested before I posted the question (except for unplugging the computer -- though I did leave it off overnight). The anti-virus/firewall issue was my first thought, but turning it all off didn't seem to help the situation.

Since it started working again, it's kept on running without a hitch -- as it has for several years with the same or similar setup. I do expect it to happen again one of these days, in which case I'll report back. Maybe eventually I'll be able to find some connection to each time it happens.

Thanks again.