Certificate

Security Certificate

Self-Signed

# cd /tmp
# SUBJ="/C=US/ST=Indiana/L=Gary/O=COMPANY/OU=IT/CN=mail.domain.tld/emailAddress=user@domain.tld"
# openssl req -x509 -newkey rsa:2048 -keyout key.pem -out cert.pem -sha256 -days 3650 -nodes -subj $SUBJ
# cat cert.pem key.pem > servercert.pem
# cp servercert.pem /var/qmail/control/servercert.pem
# remove *.pem

Let's Encrypt

(assumes a functioning Apache web server for domain.tld)

# dnf -y install certbot python3-certbot-apache
# certbot --apache -d domain.tld -d mail.domain.tld --rsa-key-size 2048 --key-type rsa

A 2048 bit key should have been created (qmail won't run with anything less), if not, force it with RSA options (in red) above.
Check the size of the private key with the following command

# openssl rsa -in /etc/letsencrypt/live/mydomain.tld/privkey.pem  -text | grep Key

Move key & certificate in place

# cp -p /var/qmail/control/servercert.pem /var/qmail/control/servercert.pem.bak
# cat /etc/letsencrypt/live/domain.tld/privkey.pem /etc/letsencrypt/live/mydomain.tld/fullchain.pem > servercert.pem
# cp ./servercert.pem /var/qmail/control/servercert.pem

GoDaddy

# cd /tmp
# SUBJ="/C=US/ST=Indiana/L=Gary/O=COMPANY/OU=IT/CN=mail.domain.tld/emailAddress=user@domain.tld"
# openssl req -new -newkey rsa:2048 -nodes -keyout key.pem -out csr.pem -subj $SUBJ
# openssl req -in csr.pem -noout -text (Examine your signing request)

Submit signing request (csr.pem) to GoDaddy, download crt and crt bundle, pack, and move in place.

# cat key.pem 7531fdb8504afe19.crt gd_bundle-g2-g1.crt > servercert.pem
# cp servercert.pem /var/qmail/control

Implementation

Restart Services

# qmailctl stop && sleep 2 && qmailctl start
# systemctl reload dovecot httpd

Renew (Let's Encrypt) *with script

# crontab -e
0 0 * * * /opt/certbot/renewcerts renew

#!/bin/bash

#   Script: renewcerts
# Function: renew certificates

LOG=/usr/command/certs/certs.log
days=3

today=`date`
today=`date --date="$today" --utc +%s`
CD=/etc/letsencrypt/live
FC=fullchain.pem
PK=privkey.pem

mailcert () {
  cat $CD/$1/$PK $CD/$1/$FC > ./servercert.pem
  cp -p /var/qmail/control/servercert.pem /var/qmail/control/servercert.pem.bak
  cp servercert.pem  /var/qmail/control/servercert.pem
  systemctl reload dovecot  
  qmailctl stop && sleep 2 && qmailctl start
}

for CDOM in `ls $CD`
do
  [ "$CDOM" = "README" ] && continue
  exp=`openssl x509 -dates -noout < $CD/$CDOM/$FC | grep notAfter | sed 's/notAfter=//'`
  off=`date --date="$exp" --utc +%s`
  diff=$(( (off - today)/86400 ))
  echo "Certificate Domain: $CDOM, Days to expire: $diff"
  echo ""
  if [ $diff -le $days ]
  then
     certbot renew --cert-name $CDOM --apache --rsa-key-size 2048 --key-type rsa
     systemctl reload httpd
     mailcert $CDOM
  fi
done

exit 0

Mail server settings

Dovecot

ssl_cert = </var/qmail/control/servercert.pem
ssl_key = </var/qmail/control/servercert.pem

Submission

#!/bin/sh
QMAILDUID=`id -u vpopmail`
NOFILESGID=`id -g vpopmail`
MAXSMTPD=`cat /var/qmail/control/concurrencyincoming`
SMTPD="/var/qmail/bin/qmail-smtpd"
TCP_CDB="/etc/tcprules.d/tcp.smtp.cdb"
HOSTNAME=`hostname`
VCHKPW="/home/vpopmail/bin/vchkpw"
export FORCETLS="1"
export SMTPAUTH="!"

exec /usr/bin/softlimit -m 128000000 \
    /usr/bin/tcpserver -v -R -H -l $HOSTNAME -x $TCP_CDB -c "$MAXSMTPD" \
    -u "$QMAILDUID" -g "$NOFILESGID" 0 587 \
    $SMTPD $VCHKPW /bin/true 2>&1

SMTPS

#!/bin/sh
QMAILDUID=`id -u vpopmail`
NOFILESGID=`id -g vpopmail`
MAXSMTPD=`cat /var/qmail/control/concurrencyincoming`
SMTPD="/var/qmail/bin/qmail-smtpd"
TCP_CDB="/etc/tcprules.d/tcp.smtp.cdb"
HOSTNAME=`hostname`
VCHKPW="/home/vpopmail/bin/vchkpw"
export SMTPS="1"
export FORCETLS="0"
export SMTPAUTH="!+cram"

exec /usr/bin/softlimit -m 128000000 \
    /usr/bin/tcpserver -v -R -H -l $HOSTNAME -x $TCP_CDB -c "$MAXSMTPD" \
    -u "$QMAILDUID" -g "$NOFILESGID" 0 465 \
    $SMTPD $VCHKPW /bin/true 2>&1