From https://developer.mozilla.org/en-US/docs/Mozilla/Thunderbird/Autoconfiguration All the lookup mechanisms use the email address domain as base for the lookup. For example, for the email address fred@example.com Is the xml file named after the email address domain? For example fred@example.com would have an XML file example.com.xml even if the mail server to be used was mail.mydomain.com Check also for case. Most of the developers use operating systems with case sensitive file systems so often Mail is not equal to mail in file names used by Thunderbird. Technically all mail server names and email addresses are lower case.