Originally Posted By: Am_Fear_Liath_Mor
I already have PGP installed on Thunderbird.

One recent revelation is that a PGP-encrypted email red flags an email message, even if the NSA can't/doesn't bother trying to decrypt it. So, even if they do actually follow some procedure to ignore/delete certain info that it collects, a PGP encrypted message and its accompanying metadata will be retained "forever."