11.14. How do I specify the hash algorithm?

As of version 0.95.0, Enigmail relies by default on GnuPG for selecting the hash (digest) algorithm. From GnuPG, the hash algorithm can be specified in the file gpg.conf using the parameter digest-algo hash_algorithm.
If you want to select the hash algorithm from within Enigmail, you can do so by modifying the preference extensions.enigmail.mimeHashAlgorithm and assigning to it one of the following values:
      0 – automatic selection, let GnuPG choose
      1 – SHA1
      2 – RIPEMD160
      3 – SHA256
      4 – SHA384
      5 – SHA512
To know more about modifying the preference values manually, also read Section 9.2.