Now the A record definitly works.
But the mails are still queued in the referred queue with the error "mail transport unavailable".
Code:
Feb 6 06:55:45 zimbra postfix/qmgr[28968]: 730339780D2: to=<kerstin.urschler_old@schuelerunion.at>, relay=none, delay=46182, delays=45985/197/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)
Feb 6 06:55:45 zimbra postfix/qmgr[28968]: 730339780D2: to=<spam@schuelerunion.at>, relay=none, delay=46182, delays=45985/197/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)
Feb 6 06:55:45 zimbra postfix/qmgr[28968]: 7EAB29782C4: to=<kerstin.urschler_old@schuelerunion.at>, relay=none, delay=8036, delays=7839/197/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)
Feb 6 06:55:45 zimbra postfix/qmgr[28968]: 76FF71A74BA3: to=<elisabeth.gehringer@schuelerunion.at>, relay=none, delay=69394, delays=69197/197/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)
Feb 6 06:55:45 zimbra postfix/qmgr[28968]: 7B31A1A74D2E: to=<elisabeth.gehringer@schuelerunion.at>, relay=none, delay=49880, delays=49683/197/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)
Feb 6 06:55:45 zimbra postfix/qmgr[28968]: 79666978267: to=<spam@schuelerunion.at>, relay=none, delay=17270, delays=17073/197/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)
Feb 6 06:55:45 zimbra last message repeated 3 times
Feb 6 06:55:45 zimbra postfix/qmgr[28968]: 7B6F59780D8: to=<katharina.auer@schuelerunion.at>, relay=none, delay=46541, delays=46344/197/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)
Feb 6 06:55:45 zimbra postfix/qmgr[28968]: 7B6F59780D8: to=<martina.kaufmann@schuelerunion.at>, relay=none, delay=46541, delays=46344/197/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)
Feb 6 06:55:45 zimbra postfix/qmgr[28968]: 7B6F59780D8: to=<sandra.spoerk@schuelerunion.at>, relay=none, delay=46541, delays=46344/197/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)
Feb 6 06:55:45 zimbra postfix/qmgr[28968]: 7B6F59780D8: to=<spam@schuelerunion.at>, relay=none, delay=46541, delays=46344/197/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)
Feb 6 06:55:45 zimbra postfix/qmgr[28968]: 7B6F59780D8: to=<spam@schuelerunion.at>, relay=none, delay=46541, delays=46344/197/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)
Feb 6 06:55:45 zimbra postfix/qmgr[28968]: 71794978201: to=<spam@schuelerunion.at>, relay=none, delay=29379, delays=29182/197/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)
Feb 6 06:55:45 zimbra postfix/qmgr[28968]: 7D5171A74C0A: to=<spam@schuelerunion.at>, relay=none, delay=66072, delays=65875/197/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)
Feb 6 06:55:45 zimbra postfix/qmgr[28968]: 7475297809F: to=<kerstin.urschler_old@schuelerunion.at>, relay=none, delay=51363, delays=51166/197/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)
Feb 6 06:55:45 zimbra postfix/qmgr[28968]: 7DAE9978205: to=<spam@schuelerunion.at>, relay=none, delay=29373, delays=29176/197/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)
Feb 6 06:55:45 zimbra postfix/qmgr[28968]: 7DD941A74AD5: to=<spam@schuelerunion.at>, relay=none, delay=76348, delays=76151/197/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)
Feb 6 06:55:45 zimbra postfix/qmgr[28968]: 7DD941A74AD5: to=<spam@schuelerunion.at>, relay=none, delay=76348, delays=76151/197/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)
Feb 6 06:55:45 zimbra postfix/qmgr[28968]: 75EEA1A74CA8: to=<spam@schuelerunion.at>, relay=none, delay=59007, delays=58810/197/0/0, dsn=4.3.0, status=deferred (mail transport unavailable)