10.11. I use a non-English character set, and my own signatures are invalid

When sending signed emails containing non-English characters (e.g. å or ð), the signature may fail to verify.
To fix this problem, make sure the option For messages that contain 8-bit characters, use 'quoted printable' MIME encoding is unchecked in your mail client. In SeaMonkey, this option is under Edit → Preferences → Mail & Newsgroups → Composition. In Thunderbird, this option is under Tools → Options → Composition.