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Should be a must have feature beside default S/MIME Rated 4 out of 5 stars
Having a S/MIME certificate is hard but having a GnuPG keypair is easy. On the other hand using OpenPGP signing without using PGP/MIME messes the email message for those who doesn't have OpenPGP compatible client.
So the best part of this add-on is "OpenPGP Signing + PGP/MIME"