Hello,
Please witch add-on possibilities exist to synchronize the Calednar with Dovecot and Postfix.
Can give me here any a possible direction ?
Postfix and Dovecot are responsible for mailing. If you want a calendar, you probably want a CalDav server or a Microsoft ActiveSync compatible server.
There are various solutions out there, I know of caldav servers and libraries from sabredav.io, calendarserver.org, agendav.org, davical.org, radicale.org and bedework at apereo.org
If you want a more integrated solution there is open-xchange (dovecot company is part of), sogo.nu, kolab.org, obm.org, horde, citadel, zimbra, zarafa
While I have not tried all of those, I can recommend sogo.nu. I have not tried all of the above, and when I evaluated some of them (back then in 2012/13), some did not meet our expectations concerning speed, ease of setup/administration and stability. But things might have changed of course.
Hope this helps, infoomatic