This is for Autocrypt Level 1 or later...
The MUA generates a strong "backup code" and gets the user to write it down somewhere. Then it serializes its secret key material into a message encrypted by the the backup code. This message is given a custom header and is sent to the account in question:
Autocrypt-Secret-Key-Backup: key_backup_data=<encrypted_secret_key> From: alice@example.net To: alice@example.net
.. todo:: should the MUA store the message in the SMA, or store it to file or what?
.. todo:: Fill in here
Prompting the user for backup code?
Note also that the backup code MUST be strong -- it is subject to brute force attacks by anyone who holds a copy.
.. todo:: say something about the relationship between backup and sync