This problem is caused by Thunderbird <i>reusing</i> existing outgoing server settings that may already exist for the same server (but different username) when setting up a new account. It doesn't tell you those settings are being <i>shared</i> across multiple accounts nor gives you the option to create new separate outgoing server settings specific to the new account during the account setup process, and hence you end up changing the outgoing server settings for all the accounts sharing those settings every time you change the settings for one of the accounts… You may create and manage multiple outgoing server settings for the same server, but cannot do it from within the main account settings. You have to do that from a separate <b>Outgoing Server (SMTP) Settings</b> pane you may open by selecting the corresponding option at the bottom of the list of accounts in Account Settings. The following article has a screenshot that shows the SMTP settings pane: https://support.mozilla.org/kb/cannot-send-messages You can add there as many settings for the same outgoing server as you want, with each entry having its own username, and then assign the right outgoing server settings to each account in the main account settings.