I use thunderbird both on a linux laptop and under windows. I exported my private openPGP keys from Thunderbird on linux and imported them into the program on windows. When I use the windows program to send encrypted emails between accounts (e.g. from my gmail account to my hotmail account) the message can be decrypted but it says it has an invalid digital signature. This does not happen when I do this under linux. Any idea how I can fix this? Many thanks!