I want to check my E-Mail accounts only manually. I disabled the syncronisation but that did not help. EM Client ignores that and checks all my mails several times. Is it possible to disable that generaly?
That setting only applies to email accounts setup as POP3, and with syncing contacts and calendars. If your account is setup as IMAP it will be permanently connected to the server.
One option, if you want to keep using IMAP, is to take the application offline when you don’t want it to sync. You can do that in Menu > File > Work Offline.