OX strores the users SMTP and IMAP server as a user setting in the MySQL DB.
You can either modify existing users with /opt/open-xchange/sbin/changeuser or (safer) trigger any modification (e.g. set the user birthday) and it will be changed.
To observe the change, first run:
ucr set ox/debug/level=4
service univention-directory-listener restart
Then modify the user and read /var/log/univention/listener.log
To verify the result run:
/opt/open-xchange/sbin/listuser -c 10 -A oxadmin -P "$(</etc/ox-secrets/context10.secret)" --csv | grep THE_USERNAME
Greetings
Daniel Tröder