The latest auto update to Enigmail, the essential plugin for Thunderbird for encrypted mail, is a fairly dynamic project that occasionally makes UI and usability decisions that not everyone agrees with.
Why? OK – PGP/MIME leaks less metainformation than inline PGP, but at the expense of compatibility. K9 should support PGP/MIME, but it doesn’t. Enigmail should have synchronized with K9 and released PGP/MIME when mobile users could use it.
But encryption people often insist that the only use case that matters is some edge case they think is critical. They like to say that nobody should read encrypted mail on a mobile device because the baseband of the device is intrinsically insecure (all cell phones are intrinsically insecure – phones should treat the data radio as a serial modem and the OS and the data modem should interact only over a very simple command set – indeed, the radio should be a replaceable module, but that gets beyond this particular issue).
For now, make sure your default encoding is Inline-PGP or you’ll break encryption. Encryption only works if it is easy to use and universally available. When people can’t read their messages, they just stop using it. This isn’t security, this is a mistake.