Here is some more debug info:
May 16 13:22:28 efa postfix/smtpd[4227]: 4QLDNh0K3LzB6HKt: client=mailoutp.awi-potsdam.de[139.75.1.113]
May 16 13:22:28 efa postfix/cleanup[4235]: 4QLDNh0K3LzB6HKt: message-id=<
050b7265188649de82b5915b078eba02@awi.de>
May 16 13:22:28 efa opendkim[2623]: 4QLDNh0K3LzB6HKt: mailoutp.awi-potsdam.de [139.75.1.113] not internal
May 16 13:22:28 efa opendkim[2623]: 4QLDNh0K3LzB6HKt: not authenticated
May 16 13:22:28 efa opendkim[2623]: 4QLDNh0K3LzB6HKt: DKIM verification successful
May 16 13:22:28 efa opendmarc[2622]: 4QLDNh0K3LzB6HKt: ignoring invalid ARC-Authentication-Results header "i=1;#012#011smtpd-out;#012#011none"
May 16 13:22:29 efa postfix/cleanup[4235]: 4QLDNh0K3LzB6HKt: milter-reject: END-OF-MESSAGE from mailoutp.awi-potsdam.de[139.75.1.113]: 4.7.1 Service unavailable - try again later; from=<
x@awi.de> to=<
y@z.net> proto=ESMTP helo=<mailoutp.awi-potsdam.de>
May 15 15:27:26 efa kernel: opendmarc[745]: segfault at 0 ip 00007f32b05d3a84 sp 00007f32ad98e118 error 4 in libbsd.so.0.11.7[7f32b05c7000+14000]
May 15 17:47:26 efa kernel: opendmarc[15643]: segfault at 0 ip 00007f0bd70ada84 sp 00007f0bcefd8118 error 4 in libbsd.so.0.11.7[7f0bd70a1000+14000]
May 16 08:57:27 efa kernel: opendmarc[10935]: segfault at 0 ip 00007fa7dfafaa84 sp 00007fa7cdfd6118 error 4 in libbsd.so.0.11.7[7fa7dfaee000+14000]
May 16 10:07:27 efa kernel: opendmarc[15674]: segfault at 0 ip 00007f9b47aeea84 sp 00007f9b456aa118 error 4 in libbsd.so.0.11.7[7f9b47ae2000+14000]
May 16 11:17:27 efa kernel: opendmarc[12461]: segfault at 0 ip 00007f747f131a84 sp 00007f747cced118 error 4 in libbsd.so.0.11.7[7f747f125000+14000]
May 16 12:27:27 efa kernel: opendmarc[25279]: segfault at 0 ip 00007f6919502a84 sp 00007f69160bc118 error 4 in libbsd.so.0.11.7[7f69194f6000+14000]
May 16 12:44:40 efa kernel: opendmarc[4205]: segfault at 0 ip 00007f35f91bca84 sp 00007f35f6d78118 error 4 in libbsd.so.0.11.7[7f35f91b0000+14000]
May 16 13:22:28 efa kernel: opendmarc[8947]: segfault at 0 ip 00007f4cfb02ca84 sp 00007f4cf8be8118 error 4 in libbsd.so.0.11.7[7f4cfb020000+14000]
May 16 14:47:28 efa kernel: opendmarc[7859]: segfault at 0 ip 00007f854d62ca84 sp 00007f85499e5118 error 4 in libbsd.so.0.11.7[7f854d620000+14000]
May 16 15:12:27 efa kernel: opendmarc[10097]: segfault at 0 ip 00007f40410eaa84 sp 00007f403e4a5118 error 4 in libbsd.so.0.11.7[7f40410de000+14000]
May 16 15:57:27 efa kernel: opendmarc[13646]: segfault at 0 ip 00007f87b58f7a84 sp 00007f87b34b3118 error 4 in libbsd.so.0.11.7[7f87b58eb000+14000]
May 16 17:07:27 efa kernel: opendmarc[16090]: segfault at 0 ip 00007fa06cf11a84 sp 00007fa0696be118 error 4 in libbsd.so.0.11.7[7fa06cf05000+14000]
May 16 17:32:28 efa kernel: opendmarc[8752]: segfault at 0 ip 00007f5ddfb25a84 sp 00007f5dcffda118 error 4 in libbsd.so.0.11.7[7f5ddfb19000+14000]
Thanks
Gerald