Reinstalling Thunderbird almost certainly won't help you since the user data are stored separately from the program itself. Your issues with multiple accounts will in all likelihood remain.

https://support.mozilla.org/en-US/kb/profiles-tb

Thunderbird doesn't "blackball" accounts. It has no knowledge upon which to base such a decision. The message means that the sending ("SMTP") server doesn't recognize the "From:" address, which is almost definitely an issue arising from your typing mistakes.

Note that if you are setting up multiple email accounts with different providers, or addresses inherited from providers who have been absorbed into your current provider, you may need to set up distinct smtp servers for each of them. As an example, I used to use an ISP called lineone. They gave me an email address that had the form user@lineone.net. Lineone was absorbed by another ISP, Tiscali who gave me another email address, user@tiscali.co.uk. So while they still supported my old lineone.net address, it had to be sent via a specific server, e.g. smtp.lineone.net and with the user@lineone.net email address as its username.

Your error message suggests that the server doesn't recognize the "From:" address as one of its own.

You delete an account via Account Settings. Select the unwanted account, and click the button at the very bottom, which may be labelled "Account Actions". (Unfortunately this button changes its name to reflect what it was used for previously, so the label isn't constant. :-( )

Since nothing much is working right now, you might do better to delete the profile and start over. I'd export the address books first. You need to export each individual address book (e.g. Personal Address Book, Collected Addresses) to its own file, preferably LDIF format. In the Address Book, select an address book (e.g. Personal Address Book) go to Tools|Export and use the file picker to choose a place, name and type for the exported address book data file. Repeat for each address book you need to preserve.

I believe it has been fixed to not allow you to try to export the virtual composite (and therefore non-existent) "All Addresses" address book.

You have given me a lot to think about, for which I am grateful. I may have a few more questions in a day or so. In the mean time, thank you.

The problem of not being allowed to send was caused by Verizon, my ISP. About Nov 16, 2016, Verizon decided to "enhance security and prevent phishing" by refusing to allow any non-local "From" address, i.e. one that doesn't say "Verizon". They've gotten a storm of howls, and are reconsidering, they say. For now, I've set Thunderbird aside and am using webmail from the company that hosts my domain.

unless verizon are actively blocking ports, you should simply be able to set up the hosted email domain as an outgoing server in Thunderbird. That is actually the default way to add an account.

So when you send mail from Me@Mydomain.com the mail server at MyDomain.com does the sending. Verizon should not have anything o do with the sending of mail for any mail account that is not a Verizon account. In refusing to send mail for domains they do not control I must applaud them. Despite your inconvenience.

As you suggest, Verizon is actively blocking ports. I have my own domain (my address could have been Me@MyDomain.com). Verizon refuses to pass on my outgoing mail because it doesn't have "Verizon" in the "From" address. Since Verizon controls my gateway to the internet, they can block me. So I can't use Thunderbird or any similar packages, unless I find a different Internet Service Provider. That is more "inconvenience" than I like. You may applaud if you wish, but how do YOU deal with such a problem?

.....

So, they block your gmail, hotmail, yahoo accounts etc? None of those would have "verizon" in the sender's address.

Is this port blocking (if so, which port?) or their smtp server sensibly rejecting foreign domains?

Good question. This how I understand their new posted policy. They say "Effective November 15, 2016, for enhanced security and phishing protection, we are eliminating the ability to use non-local 'from' addresses in conjunction with your Verizon.net email account." BUT my wife uses a gmail.com address, and we connect to the internet with the same Verizon account. I'll have to check how that works.

"…eliminating the ability to use non-local 'from' addresses in conjunction with your Verizon.net email account…" i.e. via their smtp server. They are not blocking access to other operators' servers e.g. gmail.

Does your domain host not provide an smtp server for you to use? Or they do, and that's the one giving problems?

The error message is "The mail server responded: 5.7.1 From address not allowed. Please verify that your email address is correct in your account settings and try again." So it sounds like I have a server, just as I did until a week ago.

If it's "sensible" to reject foreign domains, how do the thousands of .com and .org addresses work? As a very small business owner with a .com address, can I use the same method or is it out of reach for someone operating out of his home?

I'm assuming that you're using verizon's smtp server. Apologies if I have misunderstood your situation.

If you have handed over money for a .com or .org domain then the registrar usually also provides webspace, a web server and ftp server to manage your website, and a suite of pop/imap/smtp servers for your email. If all this is in place, you wouldn't need to use your ISP's smtp server.

Many years ago when we all used dial-up, it was common for an ISP to silently accept and process outgoing "non-local" email addresses. You'd set up a single smtp server in your email client and use it for all of your outgoing email. Thunderbird's separation of the smtp settings from incoming pop/imap settings reflects this pattern of use.

I can remember up to 3 or 4 years ago my own .org.uk webhost/registrar expected me to be able to find an smtp server as he didn't then provide one. He now does, reflecting this kind of tightening up across the industry. Indeed, my current ISP (bt.com) have had a "no relay" policy in place for over 10 years.

Of course, yahoo/gmail/hotmail changed it all by offering both incoming and outgoing servers so you didn't fall foul of trying to "relay" non-local addresses via your ISP's smtp. ISPs refusing to relay is now commonplace; we expect mail providers to offer the whole package.

Look at your gmail settings and you'll see they send via smtp.gmail.com; they don't ask anything of verizon's smtp server. I would hope that whoever provides your domain can also offer you a smtp server. If he can't include one then time to move to another who can.

My understanding of Verizons port blocking is they only block port 25 on residential services. A very reasonable situation given port 25 is supposed to be server to server communications and is really only used in a domestic situation be spam bots.

So lets dig a little deeper. what is your domain? Host? I know of at least on hosting provider (godaddy) that offers outgoing mail on port 80. The same as web browsing to bypass ISP's that actively try and block email.

Zenos, I marked your last comment as helpful, but I want to thank you personally for explaining how things work in the 21st century. This will solve a lot for me.