Sorry for spamming but looks like I'm almost done. At least no unexpected behaviour anymore. To sum it up:
1. MailEnabled checkbox not loading looks like a bug to me. Other fields like "mailAlias", "mailHomeDirectory" or even "mailAlias" work as expected. I can save a value and it will be loaded into the filed after entering the details view.
2. "mailAlias" is an interesting attributeType. It is used in both objectClases "postfixBookMailAccount" and "postfixBookMailForward": - https://github.com/leonidas-o/freeipa-postfixbook-plugin/blob/e0943c52fa5fba...
- https://github.com/leonidas-o/freeipa-postfixbook-plugin/blob/e0943c52fa5fba...
Does that mean for me, I have to use "add_missing_object_class(...)" for both objectClasses in its mailalias.py file? - https://github.com/leonidas-o/freeipa-postfixbook-plugin/blob/e0943c52fa5fba...
3. As "mailAlias" is mandatory according to the schema (see second link above, line 95 in "postfixBookMailForward" objectClass), how can I let it be generated like the regular "mail" field is generated? For example, "mail" is using "uid@domain", I think it would make sense to use "firstname.lastname@domain" for "mailAlias". I couldn't find the source code for the "mail" generation process.
4. Is there a way to say "mailAlias" values has to be unique across all users?