I have tried several machines and several accounts with same result. I can send email using same settings on my phone ( via the same router). Started mid afternoon 5/21 no changes made by me prior to that.

Thanks for your input, Flybob; I am sorry you are experiencing the same problem. It's frustrating, all right.

My AOL profile is still not working, when it was working so smoothly before. I'm beginning to suspect the Mail.com profile made a mess of things. (Yet why should that be? Aren't profiles supposed to be independent of one another?)

Let's hope a knowledgeable hero will fly to the rescue, here.

I tried disabling (Avast) anyway, and it made no difference

Try to start Windows in safe mode with networking enabled. - Win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode - Win8 https://support.microsoft.com/en-us/help/17076/windows-8-startup-settings-safe-mode - Win7 https://support.microsoft.com/en-us/help/17419/windows-7-advanced-startup-options-safe-mode#start-computer-safe-mode=windows-7

Still in Windows safe mode, start Thunderbird in safe mode. - https://support.mozilla.org/kb/safe-mode

Does the problem go away?

I am able to send, again, emails through the browser method of Mail.com

Webmail infrastructure is independent from POP/IMAP/SMTP servers. Therefore, if webmail works, this isn't an indication that there are no other problems.

I was surprised to read the "(32-bit)" notation, because I don't understand why I wouldn't have installed the correct 64-bit version.

There is no supported 64-bit Thunderbird version for Windows (yet).

Could that have anything to do with it?

No.

If I have to reinstall, I am worried that all of the stored emails/folders/information may not return.

Profile files are stored separately from the Thunderbird program files. The program files are static and do not change (except when you upgrade the Thunderbird application version). Because we store profile and program files separately, you can uninstall Thunderbird without losing your messages and settings, and if something goes wrong with a Thunderbird update your information will still be available. It also means that you don't have to reinstall Thunderbird in order to delete your information or troubleshoot a problem.

Aren't profiles supposed to be independent of one another?

They are. But I'm not sure why you choose to set up a second account in a different profile. It's quite common to set up multiple accounts in the same profile. There may be reasons to use multiple profiles though.

I am seeing the same thing on all computers as of yesterday afternoon. One interesting side note. I logged into my router yesterday to check IP address and it was looking for a certificate that has never happened before.

I tried changing the logon info to another mail account I have and it does the same thing. Again I get mail fine.

flybob, please start a new topic for your problem. https://support.mozilla.org/questions/new

Dear Christ1,

Would you please accept my gratitude for taking the trouble to try and resolve this problem? I appreciate it very much.

I was also very impressed with how you tackled each of my individual "side" questions. Thank you for those replies, as well.

If I may quickly dispatch with one of the questions you asked, before moving on to the crux of the matter: "They are. But I'm not sure why you choose to set up a second account in a different profile." (In regard to my revelation as to why I had set up an individual profile for Mail.com.)

The answer: At first I set up the Mail com account as an additional account in Thunderbird, co-existing with the AOL account. When I was on the phone with the Mail.com tech rep and he performed a "computer assist" to see what we could do to get the Mail.com account to work, he suggested to make each account independent. I figured he knows something I don't, and I took him up on his suggestion.

(It probably made things worse, as the AOL account used to work for "Send" while coexisting with the Mail.com account. Once I moved Mail.com to its separate profile, neither worked.)

So..! I was very excited to follow your suggestion. I entered Safe Mode, and...

The problem with the AOL profile persisted!

Yet then, a surprise:

I tried with the Mail.com profile, and...

It went through!

(I was shocked. That was the problematic one.)

Then I performed the real test, as the Mail.com account SOMETIMES did work earlier, when I sent individual emails. Yet my real purpose is eventually to send bulk mails, and when I tried with a handful of multiple email recipients...

They went through, too! (Yeow!)

I hope you will have ideas as to how to proceed from here.

I'm also delighted you are detail oriented. There appears to be a better chance, if you have further suggestions, that you will be "spelling out" exactly what steps I would need to take, and not leave to chance that I will necessarily understand what you may be referring to.

Thanks, Christ1; I'm not the religious type, but I am feeling a certain chill that, as your namesake, you could wind up being my savior.

EDIT: After rebooting the computer, the "Mail.com" profile appears to be working, which is unbelievable. I had no idea just going to Safe Mode would fix anything, I thought that was a way to sort out problems, and then see if they could get fixed. Especially if it continues behaving, thank you so much..!

I hope you have a suggestion for the AOL one, because for that one to continue working again with Thunderbird would be more essential for me. Thank you again, Christ1.

EDIT AGAIN: I tried a second time with a multiple email list with "Mail.com," and got the same "mailbox unavailable" message. So... it would appear the problem is unresolved, alas. .

... the Mail.com tech rep ... suggested to make each account independent.

Mail accounts from different email providers are independent from each other when set up in the same profile. In your case there does not seem to be any specific reasons to use different profiles. In fact that makes it very cumbersome to check mail for both accounts.

I entered Safe Mode, and... The problem with the AOL profile persisted!

Let's get to the AOL account later on.

I tried with the Mail.com profile, and... It went through!

Windows safe mode disables anti-virus software. Anti-virus software is known to cause problems with Thunderbird.

These are some generic suggestions to avoid problems with anti-virus software.

Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

Keep it working. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

And last but not least, backup your Thunderbird profile on a regular basis. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

For any questions wrt your specific anti-virus software you should contact their support.

I tried a second time with a multiple email list with "Mail.com," and got the same "mailbox unavailable" message.

I'm not surprised. Safe mode it not the cure, it's merely a troubleshooting step.

All right then! The only way I could figure out how to make my Avast anti-virus stop being friends with Thunderbird was to add this "exception" to the Avast controls:

C:\Program Files (x86)\Mozilla Thunderbird\thunderbird.exe

(I don't know if you're familiar with Avast, but if there is more to achieving what you have suggested, I'd appreciate your letting me know. I read the links you provided -- thanks for them -- and I couldn't find other clues.)

(This is normally a "dangerous" thing to do, it seems, as incoming mails will no longer be scanned. This problem is not as worrisome for me, because I'm using Thunderbird mainly for outgoing mails.)

I sent a test mail with multiple recipients, and Mail.com cooperated. Since the tests have proven to be erratic (one time it works, next time it doesn't), I hope you won't be impatient if another curve is thrown, with more experimentation, or when I start using Mail.com for real.

The AOL is still on the blink, unfortunately.

I don't understand; the AOL profile was working so reliably, before I mixed things up with Mail.com. Yet as you have made clear, the accounts/profiles are independent of each other, so what difference should the addition of Mail.com have made.

In my beginning post, I mentioned a solution that I thought was the "jackpot" ("Try recreating the password again"); I wonder why those two users had such success with this solution for the exact same problem, and it did not make a difference in my case. (I know, I know, each case has its own peripherals, and cure-alls are never guaranteed; it's just that this particular solution sounded so... promising!)

Thank you very much, Christ1. .

there re some notes on avast here https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#AVAST

Some a old, but it might give you ideas as to the types of issues avast has been known to cause specifically.

The only way I could figure out how to make my Avast anti-virus stop being friends with Thunderbird was to add this "exception" to the Avast controls: C:\Program Files (x86)\Mozilla Thunderbird\thunderbird.exe

You'll need to create an exception for the profile folder as described above, not for the program folder.

Thank you very much for your input, Matt.

Christ1, here is the point where I hope you will have more patience with me. I certainly perused what you had written earlier.

Your relevant statement from your earlier post was this:

"Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. http://kb.mozillazine.org/Profile_folder_-_Thunderbird"

I did go to that link, but all it described was how to find the Profile. (In my case, "%APPDATA% > "Roaming" > Thunderbird → Profiles.") The page went on to describe what each file in the Profiles folder signified, and I didn't spot anything else that seemed to be relevant to our situation.

The only thing I could figure out to do at that point was to go to Avast, and once there, the only way I could figure out how to create an exception was to add that "C:\Program Files (x86)\Mozilla Thunderbird\thunderbird.exe" path to Avast's "Exceptions."

Would you kindly be more specific, as to what I would need to do, in a more step-by-step manner? (Sorry for not being on the ball, here.)

By the way, as an experiment, I decided to "disable all shields" from Avast, and then I tried to send an email from the AOL profile. That silly "mailbox unavailable" message rudely showed up, once more.

Thank you very much.

We are not even users of avast, so making specific instructions is not something I can do. Avast provide instructions here https://www.getavast.net/support/managing-exceptions

create an exception for your profile which you incorrectly identify as being. %APPDATA% > "Roaming" > Thunderbird → Profiles

To get the correct path. Do the following. windows key +R type %appdata% and press enter.

Windows explorer will open and in the bar at the top will show the path as used on your computer for %appdata% and this is a windows environment variable, it is not a "real" location. Only something windows can be expected to reliable convert to a real location based on windows version and user account information.

Note that clicking into the link in the bar will convert it from the "friendly version windows often displays to the one with the \ separator that avast wants.

Thanks, Matt; I appreciated the trouble you went through.

I had only mentioned that %APPDATA% business to point out the portion of the page Christ1 referred me to (that would be http://kb.mozillazine.org/Profile_folder_-_Thunderbird), just to say that page didn't tell us how to create an exception; it just pointed out where to locate the profile.

Left to my own resources (sob!) the closest path I could figure out to add as an exception to Avast was: "C:\Program Files (x86)\Mozilla Thunderbird\thunderbird.exe"

Thanks to you, I see the correct path needed to be:

C:\Users\(ME)\AppData\Roaming\Thunderbird\Profiles

I've added this as an exception.

I still have to test my Mail.com with multiple emails, but for the few emails I've been testing with, so far, so good.

The AOL account is still giving me the "Mailbox unavailable" message, unfortunately.

Thank you so much. .

What authentication method are you using with AOL? If your not already, try oAuth.

See also https://support.mozilla.org/en-US/questions/1259715

Thanks, Matt.

Of course, this is the nightmare for those who develop a problem, and then seek a solution to the problem by consulting wonderful and helpful people as yourself, and Christ1.

That came out of left field. What the heck is an "OAuth"?

Went on the Web to try and learn more. Evidently, when Verizon/AOL/Yahoo got together, they created this "OAuth" business, apparently in an attempt to get people to use their apps, or something. (A search result also claimed they got rid of "OAuth." Okay.)

That link you provided wasn't directly helpful, Matt, at least as far as I was able to make heads/tails of it, and seemed to be proprietary to ATT customers, so I searched for "OAuth" in this very TB forum, and this page opened up:

"https://support.mozilla.org/en-US/questions/1259692

As of this writing, there were 12 results for the mysterious "OAuth," and then I began the lovely process of trying to make sense of it. Must admit, very time-consuming and also upsetting; what is the need to be forced to learn all of this when everything was working so well before.

(Before I embarked on this learning curve, I performed a System Restore to see if AOL can start working as it did earlier. No dice.)

So to help others, here is what I found. First, the word "APP" is being used to describe this matter, even though all I'm working with is my computer.

I went into TB's settings, by right-clicking on the AOL account. Next, I clicked on "Edit SMTP Server." When the little window opened, I changed the authentication method from "Normal password" to "OAuth2."

Suddenly a window opened directing me to AOL's browser login page. It told me that I will need to allow for Thunderbird to do the OAuth thing. Okay.

It then asked for a name for the "APP," and then I needed to click on "Generate," which created a "password" of 16 letters. Then these instructions appeared:

1) Go to the settings section of your app 2) Find the screen where you enter your AOL account username and password 3) Copy the app password above and paste it into the password field

Next, I had to figure out where TB stores the password, and then after fishing around in "Settings," I remembered the way to do that was through TOOLS > OPTIONS > SECURITY> PASSWORDS> SAVED PASSWORDS.

While there, I clicked on "Show password," and a much longer string of letters and numbers appeared, which was confusing.

(By the way, while on TB's "Account Settings," I also clicked on the next category on the left-hand pane, called "Server Settings." The authentication method there was still at "Normal password," so I changed this to "OAuth2.")

Then came time for the test. I sent an email.

"Mailbox unavailable."

Then I wondered whether I will be able to access my AOL account, now that this "OAuth2" business possibly gummed up the works. My old password was needed to get in.

Then a strange thing happened. It asked for the "OAuth2" business to get generated once more, and now I have a whole new string of a 16 letters.

Naturally, that was unnecessarily flummoxing. I'm now short two hours of life, and the problem remains. .

Went on the Web to try and learn more. Evidently, when Verizon/AOL/Yahoo got together, they created this "OAuth" business

No, they created 'Oath'. That has got nothing to do with 'OAuth' or 'OAuth2'.

Then came time for the test. I sent an email. "Mailbox unavailable."

I'd suggest you follow Matt's instructions above. https://support.mozilla.org/en-US/questions/1259692#answer-1225024

Thanks, Christ1; and welcome back.

I see I confused 'Oath' with 'OAuth' ; thanks for pointing that out.

As described in my last post, I did follow Matt's kind suggestion.

The crucial link from Matt's link pertained to ATT (https://www.att.com/esupport/article.html?source=ESSZ0SSPR00T1EsEM&wtExtndSource=20190320235353_AT%26T+Internet+Email+%26+Security_Wireline_LITHIUM_2201311638#!/email-support/KM1240308?gsi=dcwqhx), and I attempted to go "OAuth" AOL-style, as best as I could manage. .

See if this helps. https://support.mozilla.org/en-US/questions/1260234#answer-1226149