The Message Filters dialog allows certain criteria to be defined and an action to be performed when that criteria is matched. For one particular filter, though, the criteria keeps disappearing. This means that the action is applied to ALL email. The OS is Windows 8.1 64-bit. The Thunderbird version is 24.5.0. I have selected the menuitem "Tools >> Message Filters". The email message filter has been configured as: Filters For: "Local Folders"; Filter name: "@hmrc.gov.uk"; Apply filter when: Manually Run (TRUE); Getting New Mail (TRUE, Filter before Junk Classification); Match all of the following (TRUE); From >> Contains >> @hmrc.gov.uk" Perform these actions: Delete Message (TRUE); I have tried to completely delete the Filter and then re-start my PC - and then add the filter again. When I checked the filter (by closing the dialog and then opening it again) the filter criteria appeared to have been accepted. However, after a short time (c. 1 day) the criteria is lost, causing ALL emails to be deleted. The drop down menus in the "Filter Rules" dialog (where the criteria is defined) are completely missing for this Filter Rule. All other filter rules are OK and are being applied successfully. I have run AVG and SuperAntiSpyware in order to try to detect any undesirable cause, so I can only assume that this is a bug in Mozilla Thunderbird. Any help/suggestions which may resolve this issue would be welcome.