Sounds like you are trying to send from an email address on server A, but actually using an SMTP out

Sounds like you are trying to send from an email address on server A, but actually using an SMTP outgoing server for a different account B.

So the SMTP server has rejected your attempt to send via B using A.

Right click on the account email address name in Folder Pane that you are using in the 'FROM' Select 'Settings' Bottom right - click on 'Edit SMTP Server' Does the 'UserName' have the same email address as the account email address ? If no, then click on Cancel to close.

Click on the drop down 'Outgoing Server SMTP' option to see if you can locate the correct SMTP for that account in the drop down list.

If having a problem knowing whether you have selected the correct one then..... In left Pane at the bottom select 'Outgoing Server (SMTP)' It will show you a list of all your outgoing smtp servers. Select each in turn to see which one has the same UserName as the account email address that you want to send from. When you locate it, select it, then click on 'Edit' button Description: type in the email address which should be the same as the 'UserName'. Click on OK

Now reselect the account name in left pane Bottom right - 'Outgoing server smtp' click on the drop down and select the one that has the same emal address as account.

If using version 68*, click on OK

Restart Thunderbird.