To diagnose problems with Thunderbird, try the following:

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

I think that's very sensible advise. Did both of them and the problem still exists. Add-ons disabled and safe mode made no difference.

Forgot to say that I tried to add an account for an email addr that is not in TB currently. TB appeared to set it up correctly. But got the same result.

Other ideas are much appreciated.

Meanwhile 1. I will try to get my ISP support to see what they get when I try to send or get messages. Also ask them if there's anything they might be doing to prevent the password request appearing in TB when I try to send/get messages. Maybe they detect I have an infected email system or something.

2. Running a full virus scan right now with MS Security Essentials.

3. Another test is to get another email program on this machine, add one of my email addresses and see if I can send/get with that.

4. ??? ............ LAST RESORT? Export all accounts and associated folders, uninstall TB, then reinstall and import the previously exported email data.

Please add the troubleshooting information to your post To find the Troubleshooting information:

• Open Help (or click on three-line-icon and select Help)
• Choose Troubleshooting Information
• Use the button Copy to clipboard to select all. Do not check box "Include account names"!
• Paste this in your post.

First of all, talked at length to the ISP. After much poking around they believe the problem is a firewall issue. Checking that is my next step.

Second, here's the troubleshooting info:

 Application Basics

   Name: Thunderbird
   Version: 38.6.0
   User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0
   Profile Folder: Show Folder

(Local drive) Application Build ID: 20160211132445 Enabled Plugins: about:plugins Build Configuration: about:buildconfig Memory Use: about:memory

Mail and News Accounts account1: INCOMING: account1, , (none) Local Folders, plain, passwordCleartext

account2: INCOMING: account2, , (pop3) pop.earthlink.net:110, plain, passwordCleartext OUTGOING: smtpauth.earthlink.net:587, plain, passwordCleartext, true

account3: INCOMING: account3, , (pop3) pop.earthlink.net:110, plain, passwordCleartext OUTGOING: smtpauth.earthlink.net:587, plain, passwordCleartext, true

account4: INCOMING: account4, , (pop3) pop.earthlink.net:110, plain, passwordCleartext OUTGOING: smtpauth.earthlink.net:587, plain, passwordCleartext, true

account5: INCOMING: account5, , (pop3) pop.earthlink.net:110, plain, passwordCleartext OUTGOING: smtpauth.earthlink.net:587, plain, passwordCleartext, true

Crash Reports http://crash-stats.mozilla.com/report/index/bp-0348bdc2-a7ff-46c4-b7b9-900ca2140803 (8/3/2014)

Extensions ImportExportTools, 3.2.4.1, false, {3ed8cc52-86fc-4613-9026-c1ef969da4c3} Lightning, 4.0.6, false, {e2fda1a4-762b-4020-b5ad-a41df1933103}

   TBTracer, 0.8.5, false, TBTracer@derdoc.info

 Important Modified Preferences

   Name: Value

     accessibility.typeaheadfind.flashBar: 0
     browser.cache.disk.capacity: 358400
     browser.cache.disk.smart_size_cached_value: 358400
     browser.cache.disk.smart_size.first_run: false
     browser.cache.disk.smart_size.use_old_max: false
     extensions.lastAppVersion: 38.6.0
     font.internaluseonly.changed: false
     font.name.monospace.el: Consolas
     font.name.monospace.tr: Consolas
     font.name.monospace.x-baltic: Consolas
     font.name.monospace.x-central-euro: Consolas
     font.name.monospace.x-cyrillic: Consolas
     font.name.monospace.x-unicode: Consolas
     font.name.monospace.x-western: Consolas
     font.name.sans-serif.el: Calibri
     font.name.sans-serif.tr: Calibri
     font.name.sans-serif.x-baltic: Calibri
     font.name.sans-serif.x-central-euro: Calibri
     font.name.sans-serif.x-cyrillic: Calibri
     font.name.sans-serif.x-unicode: Calibri
     font.name.sans-serif.x-western: Calibri
     font.name.serif.el: Cambria
     font.name.serif.tr: Cambria
     font.name.serif.x-baltic: Cambria
     font.name.serif.x-central-euro: Cambria
     font.name.serif.x-cyrillic: Cambria
     font.name.serif.x-unicode: Cambria
     font.name.serif.x-western: Cambria
     font.size.fixed.el: 14
     font.size.fixed.tr: 14
     font.size.fixed.x-baltic: 14
     font.size.fixed.x-central-euro: 14
     font.size.fixed.x-cyrillic: 14
     font.size.fixed.x-unicode: 14
     font.size.fixed.x-western: 14
     font.size.variable.el: 17
     font.size.variable.tr: 17
     font.size.variable.x-baltic: 17
     font.size.variable.x-central-euro: 17
     font.size.variable.x-cyrillic: 17
     font.size.variable.x-unicode: 17
     gfx.blacklist.direct2d: 3
     gfx.blacklist.direct3d11angle: 3
     gfx.blacklist.dxva: 3
     gfx.blacklist.layers.direct3d10: 3
     gfx.blacklist.layers.direct3d10-1: 3
     gfx.blacklist.layers.direct3d11: 3
     gfx.blacklist.layers.direct3d9: 3
     gfx.blacklist.layers.opengl: 3
     gfx.blacklist.stagefright: 3
     gfx.blacklist.webgl.angle: 3
     gfx.blacklist.webgl.msaa: 3
     gfx.blacklist.webgl.opengl: 3
     gfx.direct3d.last_used_feature_level_idx: 1
     mail.openMessageBehavior.version: 1
     mail.winsearch.firstRunDone: true
     mailnews.database.global.datastore.id: fd6cd457-9be4-4da5-80b9-d5e60780427
     mailnews.database.global.views.conversation.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false…
     network.cookie.cookieBehavior: 1
     network.cookie.prefsMigrated: true
     network.predictor.cleaned-up: true
     places.database.lastMaintenance: 1455506161
     places.history.expiration.transient_current_max_pages: 103155
     plugin.importedState: true
     print.print_printer: \\BPERKINS\Canon MP470 series Printer
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_bgcolor: true
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_bgimages: true
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_command:
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_downloadfonts: false
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_edge_bottom: 0
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_edge_left: 0
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_edge_right: 0
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_edge_top: 0
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_evenpages: true
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_footercenter:
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_footerleft: &PT
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_footerright: &D
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_headercenter:
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_headerleft: &T
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_headerright: &U
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_in_color: true
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_margin_bottom: 0.5
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_margin_left: 0.5
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_margin_right: 0.5
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_margin_top: 0.5
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_oddpages: true
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_orientation: 0
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_pagedelay: 500
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_paper_data: 1
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_paper_height: 11.00
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_paper_size: 0
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_paper_size_type: 0
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_paper_size_unit: 0
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_paper_width: 8.50
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_reversed: false
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_scaling: 0.50
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_shrink_to_fit: true
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_to_file: false
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_unwriteable_margin_bottom: 0
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_unwriteable_margin_left: 0
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_unwriteable_margin_right: 0
     print.printer_\\BPERKINS\Canon_MP470_series_Printer.print_unwriteable_margin_top: 0
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_bgcolor: false
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_bgimages: false
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_colorspace:
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_command:
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_downloadfonts: false
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_duplex: -1574483314
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_edge_bottom: 0
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_edge_left: 0
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_edge_right: 0
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_edge_top: 0
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_evenpages: true
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_footercenter:
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_footerleft: &PT
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_footerright: &D
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_headercenter:
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_headerleft: &T
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_headerright: &U
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_in_color: true
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_margin_bottom: 0.5
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_margin_left: 0.5
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_margin_right: 0.5
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_margin_top: 0.5
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_oddpages: true
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_orientation: 0
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_page_delay: 50
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_paper_data: 1
   print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_paper_height: 11.00
print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_paper_name:
     print.printer_\\d3ng9nf1\Canon_MP470_series_Printer.print_paper_size_type: 0

Eureka! The Windows 7 firewall is blocking TB. For a gross test I disabled it completely and presto chango, now TB send/get works. I'll need to study up on the firewall and find out how to properly use it. Thanks to my ISP, Earthllink.net, support people for pointing me in that direction. And thanks to y'all for your ideas!