This just happened some hours ago:
2017-09-06 12:40:53 H=(bastion.fedoraproject.org) [209.132.181.2] sender verify
defer for <devel-bounces(a)lists.fedoraproject.org>: host lookup did not complete
2017-09-06 12:40:53 H=(bastion.fedoraproject.org) [209.132.181.2]
X=TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256 CV=no
F=<devel-bounces(a)lists.fedoraproject.org> temporarily rejected RCPT
<kb(a)kbojens.de>: Could not complete sender verify
Well:
$: host bastion.fedoraproject.org
Host bastion.fedoraproject.org not found: 2(SERVFAIL)
Is there a problem with its DNS entry?