This problem is caused by you inadvertently using the same outgoing server settings for several accounts that each requires its own settings. This is mostly Thunderbird's fault, not yours, in that when you setup a new account, Thunderbird <i>reuses</i> existing outgoing server settings that may already exist as a result of having previously created another account for the same server (but different username / email address), without telling you those settings are being shared across multiple accounts nor giving you the option to create new separate outgoing server settings specific for this account during the account setup process. Thunderbird lets you create and manage multiple outgoing server settings for the same server, though, just not from within the main account settings. You have to do that from a separate <b>Outgoing Server (SMTP)</b> settings 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 illustrates what I'm talking about: https://support.mozilla.org/kb/cannot-send-messages You can add there as many settings for the same outgoing server as you need, with each entry having its own username, and then assign the right outgoing server settings to each account in the main account settings. This can be confusing and threads discussing this issue tend to be a bit messy. Let's see if we can make a shorter and clearer one this time… :)