check your settings because error says "Please ensure your email address and authenticating user match when sending messages" That implies your 'From' might says correct email address, but that's not the email address being used in the server settings. * Menu icon > Account Settings * Left Pane - near the bottom below Local Folders - locate and click on 'Outgoing Server (SMTP)' On the right it will display a list of outgoing servers. There should be one for each mail account. * click on each one in turn and it displays info below * You need to locate/select the one that has 'User Name' = full email address - same as the problem account. * Then click on 'Edit' If you cannot find one then you need to create one - click on 'Add' Whether you are editing or adding - use the following info: * Under 'Description' : please type the full email address of the account - note this should be the same full email address as shown in 'User Name' * Make sure the 'User Name' also has the full email address. I could advise on other server settings but I would need to know what comes after the @ in your email address and what server settings you are using now. * Click on 'OK' Now check that mail account is using that smtp. * In Left Pane select name of problem account * Bottom Right you will which 'Outgoing Server (SMTP) server it is using. * DO NOT click on 'Edit SMTP_Server' button - leave it alone. * Click on the drop down option and select the one that shows the email address of same account. Now Exit Thunderbird and allow a few moments for background processes to complete. Start Thunderbird.