A WDSEML file is a file created by Mozilla Thunderbird, a free email and news program. It contains email message information and enables Windows search to find specific content in the emails when performing Windows searches.

WDSEML files are created when the user chooses to enable Windows search integration in Thunderbird (Tools → Options, then select Advanced → General → System Integration → Allow Windows search to search messages). You can turn off the generation of these files by unchecking the option. Users will most likely never encounter WDSEML files since they are not meant to be manually opened by users. Instead, the files are referenced by Thunderbird when a user searches for information in emails. Deleting a WDSEML file does not delete the email message in Thunderbird, it only removes the search information for the message. This makes it more difficult for a user to search for the message but the message will still exist in Thunderbird. Also, deleting a message in Thunderbird does not delete the WDSEML file that contains its search information. The WDSEML files that store search information for deleted email messages can be deleted. NOTE: WDSEML stands for "Windows Desktop Search Email."