protocols: Unknown protocol: sieve

[luckydog xf]

Hi, list,

    I downloaded dovecot-2.3-pigeonhole-0.5.5.tar.gz and installed it,
after I enabled

#/etc/dovecot/conf.d/20-managesieve.conf
protocols = $protocols sieve

it said " protocols: Unknown protocol: sieve"

What's wrong?"

Thanks,
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