Postfix log:
23.08.2021, 15:06:53 info connect to mx01.emig.gmx.net[212.227.17.5]:25: Connection timed out
23.08.2021, 15:06:53 info 01F1C275A2E: to=<testmail@gmx.net>, relay=none, delay=16990, delays=16930/0.03/60/0, dsn=4.4.1, status=deferred (connect to mx00.emig.gmx.net[212.227.15.9]:25: Connection timed out)
23.08.2021, 15:06:53 info connect to mx00.emig.gmx.net[212.227.15.9]:25: Connection timed out
23.08.2021, 15:06:53 info 5BB57275A3B: to=<testmail@gmx.net>, relay=none, delay=90226, delays=90166/0.02/60/0, dsn=4.4.1, status=deferred (connect to mx01.emig.gmx.net[212.227.17.5]:25: Connection timed out)
23.08.2021, 15:06:53 info connect to mx01.emig.gmx.net[212.227.17.5]:25: Connection timed out
23.08.2021, 15:06:23 info connect to mx00.emig.gmx.net[212.227.15.9]:25: Connection timed out
23.08.2021, 15:06:23 info connect to mx01.emig.gmx.net[212.227.17.5]:25: Connection timed out
23.08.2021, 15:06:23 info connect to mx00.emig.gmx.net[212.227.15.9]:25: Connection timed out
23.08.2021, 15:05:53 info 01F1C275A2E: from=<contact@domainname.com>, size=642, nrcpt=1 (queue active)
23.08.2021, 15:05:53 info 5BB57275A3B: from=<contact@domainname.com>, size=903, nrcpt=1 (queue active)
23.08.2021, 15:05:16 info C92A2275A53: to=<testmail@gmx.net>, relay=none, delay=60, delays=0.11/0.03/60/0, dsn=4.4.1, status=deferred (connect to mx00.emig.gmx.net[212.227.15.9]:25: Connection timed out)
23.08.2021, 15:05:16 info connect to mx00.emig.gmx.net[212.227.15.9]:25: Connection timed out
23.08.2021, 15:04:46 info connect to mx01.emig.gmx.net[212.227.17.5]:25: Connection timed out
23.08.2021, 15:04:16 info disconnect from mailcowdockerized_sogo-mailcow_1.mailcowdockerized_mailcow-network[172.22.1.248] ehlo=1 auth=1 mail=1 rcpt=1 data=1 quit=1 commands=6
23.08.2021, 15:04:16 info C92A2275A53: from=<contact@domainname.com>, size=897, nrcpt=1 (queue active)
23.08.2021, 15:04:16 info C92A2275A53: message-id=<44-61239d00-7-31c18b80@58194997>
23.08.2021, 15:04:16 info C92A2275A53: replace: header Received: from 0d99d4497f23 (mailcowdockerized_sogo-mailcow_1.mailcowdockerized_mailcow-network [172.22.1.248])??(Authenticated sender: contact@domainname.com)??by mail.domainname.com from mailcowdockerized_sogo-mailcow_1.mailcowdockerized_mailcow-network[172.22.1.248]; from=<contact@domainname.com> to=<testmail@gmx.net> proto=ESMTP helo=<0d99d4497f23>: Received: from [127.0.0.1] (localhost [127.0.0.1]) by localhost (Mailerdaemon) with ESMTPA id C92A2275A53??for <testmail@gmx.net>; Mon, 23 Aug 2021 15:04:16 +0200 (CEST)
23.08.2021, 15:04:16 info C92A2275A53: client=mailcowdockerized_sogo-mailcow_1.mailcowdockerized_mailcow-network[172.22.1.248], sasl_method=PLAIN, sasl_username=contact@domainname.com
23.08.2021, 15:04:16 info connect from mailcowdockerized_sogo-mailcow_1.mailcowdockerized_mailcow-network[172.22.1.248]
Results from https://www.checktls.com/TestReceiver:
MX (10) mail.domainname.com
A-->domainname.com XX.XX.XX.XXX (primary server IP)
STS no _mta-sts.domainname.com DNS record
TLSRPT no _smtp._tls.domainname.com DNS record found
A-->mail.domainname.com XXX.XXX.XX.XX (mailserver IP)
primary mail.domainname.com
primary-->type MX
primary-->DNSSEC? no
policy-->error could not retrieve policy: 500 Can't connect to mta-sts.domainname.com:443 (Name or service not known)
mail.domainname.com MTA-STS not OK