Qmail Dovecot Authentication

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To authenticate Qmail Submssion/SMTPS against Dovecot, instead of vpopmail, download and compile [this] file

# wget https://raw.githubusercontent.com/qmtoaster/source/main/qdovauth.c
# gcc -o qdovauth qdovauth.c
# mv qdovauth /home/vpopmail/bin
# chown vpopmail:vchkpw /home/vpopmail/bin/qdovauth
# chmod 755 /home/vpopmail/bin/qdovauth

Add services to Dovecot configuration file

service stats {
    unix_listener stats-reader {
    user = vpopmail
    group = vchkpw
    mode = 0660
    }
    unix_listener stats-writer {
    user = vpopmail
    group = vchkpw
    mode = 0660
    }
}

service auth {
   unix_listener auth-qmail {
   mode = 0600
   user = vpopmail
   group = vchkpw
  }
}

# systemctl restart dovecot

Change Submission & SMTPS run files:

#VCHKPW="/home/vpopmail/bin/vchkpw"
VCHKPW="/home/vpopmail/bin/qdovauth"

# qmailctl stop
# qmailctl start

You should be good to go.

Check for success (or failure) in /var/log/maillog by the addition this export:
export QDOVAUTH_LOG=""


...qdovauth[210722]: passdb: user@domain.tld auth succeeded:IP:127.0.0.1
or
...qdovauth[210722]: passdb: user@domain.tld auth failed:IP:127.0.0.1

*Email Client: Connection Security:   SSL/TLS
               Authentication Method: Normal Password