Migrate

On destination host
Install Rocky 8 or 9
qmailctl stop && systemctl stop dovecot

On source host Source host most likely EL7
# dhost=192.168.1.3 Change IP address for your needs
# ssh-keygen
# ssh-copy-id $dhost
# ssh $dhost

First rsync email db
# rsync --progress -zv -are ssh /home/vpopmail/domains/ root@$dhost:/home/vpopmail/domains

Stop services for 2nd rsync
# qmailctl stop && systemctl stop dovecot

Second rsync subsequent changes to email db
# rsync --progress -zv -are ssh /home/vpopmail/domains/ root@$dhost:/home/vpopmail/domains

Bring over the rest of the data
# mysqldump -u root -p vpopmail > vpopmail.sql
# scp vpopmail.sql root@$dhost:/root
# rsync --progress -zv -are ssh /var/qmail/control/ root@$dhost:/var/qmail/control
# rsync --progress -zv -are ssh /var/qmail/users/ root@$dhost:/var/qmail/users
# rsync --progress -zv -are ssh /etc/tcprules.d/ root@$dhost:/etc/tcprules.d
# rsync --progress -zv -are ssh /usr/share/toaster/include/admin.pass  root@$dhost:/usr/share/toaster/include
# rsync --progress -zv -are ssh /usr/share/toaster/include/admin.htpasswd  root@$dhost:/usr/share/toaster/include
# [ ! -d /etc/mail/spamassassin ] && sqadj= || sqadj=mail/
# rsync --progress -zv -are ssh /usr/share/squirrelmail/plugins/ root@$dhost:/usr/share/squirrelmail/plugins
# rsync --progress -zv -are ssh /etc/${sqadj}spamassassin/.spamassassin/ root@$dhost:/etc/mail/spamassassin/.spamassassin
# rsync --progress -zv -are ssh /etc/${sqadj}spamassassin/*.cf root@$dhost:/etc/mail/spamassassin
# rsync --progress -zv -are ssh /var/lib/squirrelmail/prefs/  root@$dhost:/var/lib/squirrelmail/prefs

Dovecot files are not necessary for EL 8/9 you may need them for settings
# rsync --progress -zv -are ssh /etc/dovecot/toaster.conf root@$dhost:/etc/dovecot
# rsync --progress -zv -are ssh /etc/dovecot/local.conf root@$dhost:/etc/dovecot

Other MySQL database you may be using
# mysqldump -u root -p dspam > dspam.sql
# mysqldump -u root -p spamassassin > spamassassin.sql
# mysqldump -u root -p roundcube > roundcube.sql
On destination host

# mysql -u root -p vpopmail < vpopmail.sql
# qmailctl cdb && qmailctl start && sleep 2s && systemctl start dovecot && toaststat

Questions, comments, suggestions, corrections...contact Eric on the QMT list