I think the following is a better guide for someone looking for a complete setup that includes an IMAP server and that can be used with regular email clients like Thunderbird:
I set up my own server more or less following the above guide, but eschewed the database in favor of plain text files. I wanted to keep things simple since I am the only user, but the above guide should scale to big enterprise setups.
I also use this guide, but I switched it to PostgreSQL instead. The recent upgrade to Trixie brought a new Dovecot with breaking changes to its configuration. That was a bit of a pain to resolve, but everything is working fine now.
https://workaround.org/ispmail-bookworm/
I set up my own server more or less following the above guide, but eschewed the database in favor of plain text files. I wanted to keep things simple since I am the only user, but the above guide should scale to big enterprise setups.