I was playing around with CalDAV/CardDAV servers in the last period and i think Radicale or BaiKal server will be great addition for directadmin. I found Radicale slow after 20k entries in addressbook. But it's normal because all the files are stored in one folder. Baikal from Sabre is using mysql, it seems to be a more convenient solution. For every email address one calendar and one addressbook should be created.