userdb unix_listen .dovecot-userdb
c = ./dovecot-auth-cli .dovecot exists username
userdb -> VERSION 1 1
userdb -> SPID 12345
userdb <- VERSION 1 1
userdb <- USER 1 username service=smtp
userdb -> USER 1 username system_groups_user=blah uid=10 gid=10
c <- yes
c wait 0