https://support.mozilla.org/en-US/kb/cannot-send-messages

Update to 38.3.0 fails

Please provide more details.

Magically, the 38.3 update loaded successfully, after failing uncountable times before with "...Please close all other copies of Thunderbird..." errors. So we can now state that I'm having this problem in TB v38.3.0.

Thunderbird knows the password, but Cox isn't getting it. Thunderbird doesn't seem to know the password, because it asks me for it with a blank line. The .jpg image I uploaded has the progression of screenshots.

You can try to delete the password Thunderbird has remembered. You'll then be prompted for the password again the next time it is needed. Also see http://kb.mozillazine.org/Setting_and_changing_email_passwords

That's also one troubleshooting step listed in the article linked above, but you probably didn't read it.

I appreciate your help.

I have deleted the entry in the passwords tab several times already. But I just did it once more, this time carefully following the close/re-open TB as requested. No changes noted.

I observe that on first prompt for password after the Send button no new record is created on the options/password tab, and on the second attempt to enter a password the field is still blank, as if no password exists, although a record is created this time with the correct password. On all subsequent password entry attempts, the password field is still blank every time, and the Options-Passwords tab continues to show correct information.

The problem appears not that TB doesn't have the right password, but rather that it doesn't seem to be presenting the password in a useful way to Cox.

My other user accounts appear to be working properly in all respects (aol and my own domain email).

I downloaded the Opera mail client, and I am able to Send with my Cox account using the SMTP settings I am using in TB (Thank you for not telling me to go there).

Do you think I should try uninstalling/reinstalling TB?

(Cox, of course, is blaming Thunderbird)

Can you post your Troubleshooting Information? At the top right of the Thunderbird window, click the menu button , then select Help, and then Troubleshooting Information.

Application Basics

   Name: Thunderbird
   Version: 38.3.0
   User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0
   Profile Folder: Show Folder

             (Local drive)
   Application Build ID: 20150928051427
   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.secureserver.net:995, SSL, passwordCleartext
     OUTGOING: smtpout.secureserver.net:465, SSL, passwordCleartext, true

   account3:
     INCOMING: account3, , (pop3) pop3.cox.net:995, SSL, passwordCleartext
     OUTGOING: smtp.cox.net:465, SSL, passwordCleartext, true

   account4:
     INCOMING: account4, , (pop3) pop.secureserver.net:995, SSL, passwordCleartext
     OUTGOING: smtpout.secureserver.net:465, SSL, passwordCleartext, true

   account5:
     INCOMING: account5, , (pop3) pop3.cox.net:995, SSL, passwordCleartext
     OUTGOING: smtp.cox.net:465, SSL, passwordCleartext, true

   account7:
     INCOMING: account7, , (pop3) pop3.longandfoster.com:995, SSL, passwordCleartext
     OUTGOING: smtp.cox.net:465, SSL, passwordCleartext, true

   account8:
     INCOMING: account8, , (pop3) pop.aol.com:995, SSL, passwordCleartext
     OUTGOING: smtp.aol.com:465, SSL, passwordCleartext, true

   account9:
     INCOMING: account9, , (pop3) pop.googlemail.com:995, SSL, passwordCleartext
     OUTGOING: smtp.googlemail.com:465, SSL, passwordCleartext, true

 Crash Reports
   http://crash-stats.mozilla.com/report/index/bp-04ec6fcd-7fec-43fc-84fc-ea1932151020 (10/20/2015)
   http://crash-stats.mozilla.com/report/index/bp-e01f6c2f-70c0-40ba-a921-e63f32141127 (11/27/2014)
   http://crash-stats.mozilla.com/report/index/bp-57b201fe-5184-47c8-9c70-b84062140627 (6/26/2014)
   http://crash-stats.mozilla.com/report/index/bp-cf33bc5f-a3a2-4e06-ac7e-bde2c2131013 (10/13/2013)
   http://crash-stats.mozilla.com/report/index/bp-1d50e2c4-5372-4018-8197-5e8192130713 (7/13/2013)

 Extensions
   ImportExportTools, 2.8.0.4, true, {3ed8cc52-86fc-4613-9026-c1ef969da4c3}
   Lightning, 4.0.3.1, true, {e2fda1a4-762b-4020-b5ad-a41df1933103}
   Auto Resize JPEG, 0.10.2, false, AutoResizeJPEG@kernresonanz.de
   AVG E-mail Scanner, 12.0.0.1892, false, avgthb@avg.com
   CompactHeader, 2.1.0, false, {58D4392A-842E-11DE-B51A-C7B855D89593}
   Recover deleted messages, 0.4, false, {8380f54c-9b7a-4107-b703-9b938e30fb54}

 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
     browser.search.update.interval: 999999999
     dom.max_chrome_script_run_time: 0
     extensions.lastAppVersion: 38.3.0
     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
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     font.name.serif.el: Cambria
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     mail.openMessageBehavior.version: 1
     mail.winsearch.enable: true
     mail.winsearch.firstRunDone: true
     mail.winsearch.global_reindex_time: 1282864012
     mailnews.database.global.datastore.id: 4a9036c2-9741-4eba-acc3-7f3de49c439
     mailnews.database.global.views.conversation.columns: {"threadCol":{"visible":true,"ordinal":"1"},"flaggedCol":{"visible":true,"ordinal":"3"},"attachmentCol":{"visible":false…
     mailnews.database.global.views.global.columns: {"threadCol":{"visible":false,"ordinal":"1"},"flaggedCol":{"visible":false,"ordinal":"3"},"attachmentCol":{"visible":fal…
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     places.database.lastMaintenance: 1445392517
     places.history.expiration.transient_current_max_pages: 103980
     places.history.expiration.transient_optimal_database_size: 166366740
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print.print_printer: Canon MX850 series Printer

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From the Troubleshooting Information there are three accounts using smtp.cox.net as SMTP server. Which account is the troubled one?

3, 5, and 7 are configured to use the same default cox smtp server. All can receive mail, none can send.

Your SMTP server settings for Cox look good.

Is there anything suspicious in the error console? Tools (Alt-T) - Error Console

This and your earlier line of questions set me on a different tack:

- How can this work for receiving mail but not for sending?
- Exactly when did Cox smtp stop working? 
- What were the associated events?
- What else does and doesn't work?
     

The Cox Curse

I downloaded and installed several other Email client programs. None could be readily configured to send mail, so I went back to Cox. The Level 2 support guy had much better tools and background to work on mail problems. I asked him if I was somehow tangled up in security. But logically, how or why could a security or password issue apply not to an account login, not to the ability to read mail, not to webmail, but only to the ability to send mail?

The Cox guy went over the account history and found an event a month ago, when Cox auto-thought I must be bogus because I accessed my Cox Email over a public network in Europe. So my problem turned out to be Cox obstructing Email sending (as if we don't have enough software trouble without otherwise intelligent entities doing these things to us on purpose, without telling anybody).

It turns out that Cox security curses the specific password in this case, and not the account, the user id, the IP address, or the user’s location. If you change to a new password, the problem disappears (With so many Cox support people unaware about this, why don’t I feel less stupid?). I had already changed the password weeks ago. But, not seeing any results because I was also working through all plausible configuration permutations at the time, I changed my password back to the original and thereby negated my chance for success. If I changed my new password back to the old one now, the problem would return.

Using Password Manager in Thunderbird for my Email accounts, I hadn’t seen or used my Cox password in quite awhile and I didn’t have it on the tip of my tongue in a faraway place. The very last thing I wanted to do on a public airport network is to guess through a lot of password possibilities until the account gets locked, where an international customer support call would cost a lot to make. And, the old password is an early casualty when you eventually try.