Besides a minor bug in QMF that sould be corrected with the next release. It is working, I’m using it regularly.
You have to import a private key on your phone or generate on the phone from CLI. Then, go to the setting app, account section. Select the email account you want to sign emails with, and see that there is a Crypto section there where you can choose the key to be used. Then, in the composer, signing the out going emails is checked by default. When sending the mail, the system will ask for the passphrase of your private key. When receiving a signed email, there is nothing to do, you will see in the header of the mail that it is signed. The verification can only proceed if you have the public of the sender on your device. If it is in a default server, then, you can download it from the UI, otherwise do it from CLI, or modify GPG configuration to add your favorite server of public keys.