Comment # 4 on bug 913092 from Andreas Stieger

(In reply to Bj�rn Voigt from comment #3)
> The next enigmail version may fix this bug. See discussion here
> https://www.thunderbird-mail.de/index.php/Thread/67342-Enigmail-1-7-Digitale-
> Unterschrift/

This should then be resolved in 1.8.1 / 1.8.2 available in mozilla:Factory now.