It's true; people often make problems worse by changing settings before asking for help. My request is to post a screenshot of your SMTP settings. Often, it's a matter of sending with no security or no password. Servers are getting tighter in wanting to verify that the sender is, indeed, a customer of the email provider. That's the first place to look. If that is not the issue, then the error message sometimes appears when there are multiple accounts for the same email provider and the incorrect SMTP server is being used. Let us know.

Thanks for the fast answer. I realize I have made my problem worse by trying to fix it??

Screen shot of SMTP settings attached - this is what they are now because this is Comcast recommendation Before it was Port 587 and SSLTLS (?) for security. I can probably put it back and get another screen shot if that would be helpful.

I have three email accounts - when I edit the SMTP settings they all change.

More information - I have three email addresses in Thunderbird - As I mentioned in my last message when I edit the SMTP information it changes to the same thing for all three.

That means two of them don't "match" and it is my understanding that they should.

How can I make them different as to user name??

What you need are Three SMTP servers in outgoing servers (SMTP) in account settings.

That is where this data you posted is drawn from

Each identity (if you look at the pane below the image you will note it offers you to add another identity under manage identities.) has its own SMTP server. Historically some US based ISP's like Comcast were rather obsessed with each user only having one avenue to send mail. So Thunderbird tended to just use any SMTP server that was already configured for the domain in question. It reduced our support burden despite what was going on not being really correct.

Scrolling forward 20 years or so and these providers have finally "got with the program" now they are requiring the username used to sign in to be the one allocated to the email address. Those older customers that have old settings get issues.

• So add the requisite SMTP servers in account settings> Outgoing servers(SMTP), give them unique names like the email they are meant to be used with. For example rename the one you are looking at to 123@comcast so you can tell from the list which it is.
• Then negotiate your three incoming accounts and make sure the identity is using the correct SMTP server to send mail.

Thank you Matt - this solves the problem AND explains why it happened. I was baffled because I have been using Thunderbird with Comcast for over 20 years!