Apr 8 14:32:15 server postfix/smtpd[23321]: Read 28 chars: RCPT TO:Exactly always 10 seconds, strange. So I looked up the error and found out, that the error is the Client reject table in the main.cf. I had there?? Apr 8 14:32:25 server postfix/smtpd[23321]: warning: 1.111.11.111.dnsbl.njabl.org: RBL lookup error: Host or domain name not found. Name service error for name=1.111.11.111.dnsbl.njabl.org type=A: Host not found, try again
smtpd_client_restrictions = reject_rbl_client sbl.spamhaus.org, reject_rbl_client blackholes.easynet.nl, reject_rbl_client dnsbl.njabl.orgas mentioned on this site then I changed to
smtpd_client_restrictions = reject_rbl_client sbl.spamhaus.org, reject_rbl_client blackholes.easynet.nlso only left the last reject_rbl_client and then it worked without the 10s delay! Not forget to decrease loglevel in /etc/postfix/main.cf back from
smtpd_tls_loglevel = 4to
smtpd_tls_loglevel = 1