Check the following first:

• Settings > Privacy & Security
• Scroll down to Passwords section
• Click on 'Saved Passwords' button

Do you see any lines that mention the 'User Name' = the account/email address that no longer exists? If yes, select the line and click on 'Remove' When all lines for non existing account are all removed - click on 'Close' Then Exit Thunderbird. It will update important files. After a few moments restart Thunderbird.

Please let me know if you found any lines which you had to remove.

If sending now works ok - please mark this comment as the chosen solution.

If you still have a problem then post the following information:

re :I have verified ALL server settings for each account as correct.

That does not necessarily mean you did it correctly. Sorry if I sound as if I do not believe you, but I've had plenty of people believe they checked and I found a discrepancy. So can you do the following because this is the only correct method when dealing with various accounts/email addresses including where there are more than one account using same server settings.

In Thunderbird

• Account Settings
• In Left pane - scroll to the bottom and below 'Local Folders' will be 'Outgoing Server (SMTP)' - click on that 'Outgoing Server (SMTP)' and the right pane will display a list of all the Outgoing SMTP servers.

You should have one smtp for each email address.

• Select the top one and click on 'Edit'
• 'Description': please type the full email address - it should be identical to the 'Username' email address.
• click on 'OK'

• Select the second smtp in list and click on 'Edit'
• Please edit the 'DEscription' field as advised previously. Obviously, each DEscription will be unique and match whatever email address is in that smtp Username' field.

• Work down through all the smtp's.
• If you find an old smtp in list for an email address/account that does not exist - please delete it.

Now you need to check each account to make sure it really is using the correct smtp server.

In Left Pane:

• Select the name of the account which is usually the email address.
• Look bottom right for the 'Outgoing Server (SMTP)'

This is where the 'Description' becomes helpful. Does it say the same identical email address as the account name you selected ? If yes, then all is ok. If no, then do the following:

• At end of DEscription text - Click on the drop down chevron icon to see drop down list and select the correct description where email address matches the account name email address.

DO NOT click on 'Edit SMTP_Server' button - if things are not correct then you end up editing the wrong smtp.

Do the same process for each mail account. In Left pane select the name of account, look bottom right and check the Outgoing Server is displaying the email address of the selected account. Correct as required. Work down through every account.

Then Exit Thunderbird. It will update important files. After a few moments restart Thunderbird

Now post the following information: In Thunderbird

• Help > Troubleshooting Information
• Scroll down to 'Mail and News Accounts' section
• Copy all the data in that section and paste it into this forum question OR upload an image file showing the info.