This might help my future self :
SELECT username, password into outfile '/tmp/passwords.txt' fields terminated by ':' optionally enclosed by '' lines terminated by '\n' from mailbox where mailbox.active = 1;
and then :
john /tmp/passwords.txt ….
root@mail:~# john /tmp/passwords.txt Created directory: /root/.john Loaded 2327 password hashes with 2326 different salts (md5crypt [MD5 32/64 X2]) Press 'q' or Ctrl-C to abort, almost any other key for status (vulnerablePassword) email@example.com