I am unable to digitally sign messages. Unfortunately, I am unable to find any issues arising other than the fact that the message simply is not signed on the receiving end. The following is all the information I thought readily helpful: * Certs are found/loaded through an OpenSC device and the certificates are reported as valid * Signing and encrypting certificates have been specified for the account without issue * When preparing a message, Digitally Sign This Message is checked and View Security info reports Yes for Digitally Signed * Upon sending the message, the error console reports only NS_ERROR_FAILURE: Trying to modify an Exchange GAL card. resource://exquilla/ewsAbDirectory.jsm line 850 * No additional items of note arise when trying to encrypt a message. It simply is not done regardless of Thunderbird saying the message will be signed and encrypted. The following software is in use: * Linux Mint 17 (Ubuntu 14.04/similar is a hard requirement) * Thunderbird 45.4.0 * ExQuilla 45.0.1161 is used and the account in question is an exchange account * OpenSC 0.13.0 with Enabled features: zlib readline openssl pcsc(libpcsclite.so.1) Thank you for the time and help