When you changed the password, did you remove the old ones from Saved Passwords in Preferences?

Does it make any difference if you include your own address in the To: box and then the recipients in the Bcc: box?

I believe that I deleted my old passwords first but I'll try again if necessary. More information: I have 2 TB accounts. Sending Bcc through my 2nd account, Ebeneezer, works just fine. I also tried your suggestion and put my own (1st acct) address in the To box and a different address in the Bcc box. This also worked fine. However, when I just put my address or someone else's address (without putting my address in To) in Bcc I get the error message.

I think some providers don't allow sending with just Bcc entries.

I checked with Cox. They report not having problems with email and they don't have restrictions against using Bcc. As I mentioned before, I have a second account that sends Bcc emails without a problem. Any other suggestions on how to fix this? Thanks.

With your two accounts, is each one sending through an smtp that has the same User Name as the sending account? In Tools/Account Settings, select an account in the left pane, then look at Outgoing Server (SMTP) in the right pane. Click Edit SMTP server... to see the User Name of the smtp and compare it with the User Name of the selected account. Even if both accounts are Cox accounts with the same settings, they should send through smtp servers with matching User Names. So, you should have 2 smtp servers.

I believe that this is what you're asking for. I've attached pics of my two accounts. They appear identical.

Your picture show that both smtp servers have the same User Name, cvincent44. If they are both Cox accounts, one smtp server should have the User Name as ebeneezer.

When I changed my Ebeneezer user name to Ebeneezer through edit smtp server I get an error message when trying to send emails through my Curtis account: "Login to server smtp.cox.net with user name Ebeneezer failed. It asks for a new password, however, when I enter my password for the email account I get the same error message again.

Let me repeat: in Tools/Account Settings, Outgoing Server (SMTP) at the bottom of the left pane, there should be 2 smtp servers for Cox. Create a 2nd one if necessary. Give each one a unique Description, e.g. cvincent44 and ebeneezer. The cvicent44 one should have cvincent44 as the User Name, and the ebeneezer smtp should have ebeneezer as the User Name.

Then select the cvincent44 account in the left pane of Account Settings and set the Outgoing Server (SMTP) in the right pane to the cvincent44 smtp. Make the ebeneezer account point to the ebeneezer smtp.

Remove the passwords from Saved Passwords in Preferences, restart TB, enter the correct password for each account when prompted.

I don't know if this problem can ever be fixed but in my case, it most definitely has nothing to do with my email provider. I have several specific distribution lists I created in Mozilla Thunderbird and they all worked using BCC for over one year. In late March, the problem surfaced for me and many others. The now ubiquitous msg is the same:

Sending of the message failed.

The message could not be sent using Outgoing server (SMTP) smtp.verizon.net for an unknown reason. Please verify that your Outgoing server (SMTP) settings are correct and try again.

IF I go and send the msg at my email provider site, AOL.com, the BCC option does work but I cannot easily replicate my specific distribution lists there and it is impractical to reenter them, so my only option is to send to all until this problem gets fixed.

I don't know if this problem can ever be fixed but in my case, it most definitely has nothing to do with my email provider. I have several specific distribution lists I created in Mozilla Thunderbird and they all worked using BCC for over one year. In late March, the problem surfaced for me and many others. The now ubiquitous msg is the same: Sending of the message failed. The message could not be sent using Outgoing server (SMTP) smtp.verizon.net for an unknown reason. Please verify that your Outgoing server (SMTP) settings are correct and try again. IF I go and send the msg at my email provider site, AOL.com, the BCC option does work but I cannot easily replicate my specific distribution lists there and it is impractical to reenter them, so my only option is to send to all until this problem gets fixed.

First, you should have your own address in the To: box and the other contacts in the Bcc: box. Second, Verizon/AOL accounts must use an app password in TB in place of the account password.

Sorry that you're still having this problem. I followed above suggestions and I think that my problem is resolved. Best of luck.