how can I migrate from an old sendmail server users in /etc/passwd with maildir mailbox and passwords in /etc/shadow to ispconfig? Is it possible and is there a howto?
Yes, this can be done. But there is no tutorial or script on that topic as far as I know. How many mail accounts are on the old server?
For this amount of users, it can make sense that you write a script using the ISPConfig remote API. You can find remote API examples in the remote_client folder of the ispconfig tar.gz file. Or you try to modify the mailbox from csv import plugin to suit your needs: https://git.ispconfig.org/ispconfig/Modules/-/tree/master/mailbox_import_from_csv
