I see that I broke the build, but I can't really understand why. Any thoughts?

Don't know about the build - only 2.0 failed on travis, on my machine it works, maybe just a local Travis CI hickup.

However correct me if I'm wrong, but your patch seems incomplete and will actually not do anything as it is now. It is not enough to add the sender's key to the :keys array, you also have to add the actual sender of the mail to the array of email addresses that ends up as the :receivers option in GpgmeHelper::encrypt. Only then the sender's key will be picked up as an encryption target.

I think the best place to add the sender to that array would be in Mail::Gpg::encrypt.

Oh, I didn't realize that. Ok, will take a look at how to rework this.