What is introducing quoted-printables into my PGP-encrypted emails?

Question

I've set up PGP email encryption on two computers using Enigmail and Thunderbird. One is running Mac OS X, and the other is running Arch Linux. Everything functions fine from within Thunderbird on both systems, but I had issues exporting email and decrypting from the command line (on the Arch system).

$ gpg2 -d foo.eml 
gpg: CRC error; F551DC - DC3F55
gpg: quoted printable character in armor - probably a buggy MTA has been used

I realised that the last few lines of the email were as follows:

CETSx/PSKdfZIP74WEciLAw=3D
=3D9VHc

It seems that quoted-printables had been introduced into the email. When I manually replaced =3D with =, gpg2 was able to decrypt the email perfectly.

I've checked both clients. Arch sends emails with =, and the Mac receives them with =. Conversely, the Mac sends emails with =3D, and Arch receives them with =3D. Both clients are using the same servers for SMTP and IMAP. Both Thunderbird clients have default settings (false) for mail.strictly_mime. The Arch client uses GnuPG 2.0.25-1, and the Mac uses GnuPG/MacGPG2 v2.0.22 (Darwin) via GPGTools.

Where could this corruption be occurring?

Answer 1

This was fixed by checking the following in the OS X Thunderbird.

Account settings > [Specific account] > OpenPGP Security > Use PGP/MIME by default.

This has the additional advantage of allowing sending of html emails, and attachments. However, support for decryption is lacking in many clients (e.g. with Android K-9 + APG).