User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.1) Gecko/20090624 Firefox/3.5 Build Identifier: version 2.0.0.22 (20090605) The address book entry allows you to select message format (Text/HTML) there should be another feature to auto encrypt all mail to this user. If Thunderbird has the users key the address book user properties could provide a check box to "Always Encrypt". If no key exist gray out the selection. Reproducible: Always Actual Results: I'm aware that you can set encryption to always ON based on email accounts however it does not provide a selection for "If available". Which would be another good solution. So your forced to have two different email accounts if you want to ensure encryption is done.