[Dovecot] what does it mean first_uid

i installed qmail + vpopmail + dovecot.i installed dovecot without problems.but i run telnet localhost 143 x login user password

it seems to error about first_uid.and i change first_uid vpopmail_uid (89) its ok.but i dont understand first_uid.and sholud i change last_uid , first_gid ,last_guid . do you recommend it

thanks everynbody

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