This is Enigmail for PowerPC Mac OS X. (Unofficial build)
Mac OS X 10.4 or later (PowerPC)
Thunderbird or compatible application (i.e. Tenfourbird) version 31 or later
Because of my bad patching, Enigmail for Tenfourbird v1.8.1 and v1.8.2 can not detect CPU name on update process. So Enigmail addon v1.8.1 and v1.8.2 is updated to G3 version. Enigmail for G3 will working fine on G4 and G5. I think you will not feel performace degradation with this issue. If you still want to use Enigmail that tuned for G4 or G5, you need to install add-on manually.
You can check integrity of files by RIPEMD160 or SHA512 hashes. It also signed by OpenPGP key of t_mrc-ct[at]users.sourceforge.jp. The public key could get from here or OpenPGP Keysever.