From the definition of wdseml files: "Each file is a partial copy (up to 49KB) of a message in the mail folder, stored in a .mozmsgs subdirectory. It is used by Windows Search Integration .... They're actually renamed .eml files. You can disable this feature by unchecking Tools -> Options -> Advanced -> General -> System Integration -> Allow Windows Search to search messages" http://kb.mozillazine.org/Files_and_folders_in_the_profile_-_Thunderbird