For the last however many months a low priority intermittent project of mine has been to add GPG's hidden recipient feature to OpenKeychain. OpenKeychain is an app which allows email encryption to be used on Android phones.
Why is hidden recipient relevant? Probably only a small minority of GPG users ever encounter or use that particular feature. The need for it arises out of an attempt to ensure that when emails are saved from that point onwards they only exist in an encrypted format and are only temporarily decrypted in RAM for viewing. If an adversary gains access to the files then they must guess or crack the GPG passphrase in order to read the content, and that buys the user time.
Emails on Freedombone are stored encrypted by default. If arriving mail is not already encrypted then a system called gpgme is used to encrypt it. To read any particular mail you then need to enter your GPG passphrase, and the email client typically remembers that for some limited amount of time (such as the 30 mins for a typical email reading session). The decryption stage makes use of the hidden recipient feature.
This means that although Freedombone emails are readable on a laptop, via Tunderbird/Icedove and Enigmail, they're usually not readable on an Android system. Some may regard this as being a feature rather than a bug, and I'd be sympathetic to that view. Android is not exactly regarded as being a secure system and so depending on your particular threat model you might want to keep email away from it, or endure the more cumbersome user interface of logging into your server via an ssh client (it works, but is not exactly a pleasant mobile user experience). However, many people do read their email on phones and so as a harm minimisation strategy if you're going to do it you might as well do it in the safest possible manner.
Getting GPG working on Android has been a long struggle. I can see complaints about not being able to read PGP/MIME formatted mail in K-9 dating back to 2011. There are closed source Android email clients which do support PGP/MIME, but source code which is not independently auditable cannot be regarded as trustworthy. Recently however the situation has definitely improved, and via OpenKeychain encrypted mail is now mostly readable on Android, with only occasional exceptions.
The patch for hidden recipient in OpenKeychain can be found here, and detailed instructions on how to compile both OpenKeychain and K-9 can be found here.
I've tested the patch fairly thoroughly, using two keys with different passphrases and it seems to work, but at present it seems doubtful that it will be accepted upstream. The feedback on the initial version of the patch was rather negative. Be that as it may, so long as hidden recipient is implemented somehow I don't really mind.