See inline comments. Other than that it looks good. Still we could improve performance for cases where value does not contain base_dn expansion macro. I think it would be good to improve Conf::expand() method so $custom could accept values of type function, then we could probably (untested) do:

This way base_dn() method is executed multiple times, it should cache the result in memory to not call domain_info() every time. I suppose we could even check if the value contains replacement macro before we call expand().

There's also managesieve_conn_options which need to be set or the code should be changed to fallback to imap_conn_options if not set?

Looks good to me. There's redundant array_unique() call, but may be fixed later.

Implemented already for all objects except mail.

Implemented support via x-custom fields.

I confirm this may happen for fields with "Generated" value and we should probably fix that. Anyway, setting type in auto_form_fields if it's already specified in form_fields is redundant. Did you find any issues when it's not set in auto_form_fields?

I think empty string should be handled as NIL, I don't know if we have this somewhere in the standard. Actually I see empty string in an annotation for the first time.

Hmm... That should not be required. Probably there's a bug in some other place.

Maybe a duplicate of https://issues.kolab.org/show_bug.cgi?id=4095.

Information in plugins/libcalendaring/README is outdated.

The shared/ handling is still wrong, but I accept this commit. Fix is pending.

Already fixed in 16460191019