If you are viewing folders and emails in account 'A' and then click on 'Write' or '+New Message' then it will automatically enter the email address of account 'A' into the 'From'. If you are viewing folders and emails in account 'B' and then click on 'Write' or '+New Message' then it will automatically enter the email address of account 'B' into the 'From'. Same is true when you click on eg: 'Reply' - it will use the email address depending upon where the email you are replying to is located. If for some reason Thunderbird is not sure which to use (this used to occur for emails in Local Folders but I've not had an issue) then it will use whatever account is set up as the default account. Perhaps this explains why you sometimes have an email address in 'From' which you would prefer not to use. There is an addon which may help your situation. 'Identity Chooser' 4.1 by thunderspeed '''Identity Chooser' helps you to always use the correct identity when composing a new email.'' Works with Thunderbird 115.0 - * * https://addons.thunderbird.net/en-us/thunderbird/addon/identity-chooser/ At the link above it does show some images so you get an idea of what occurs. How to install an addon: * https://support.mozilla.org/en-US/kb/installing-addon-thunderbird