I just tried sending myself an email to my gmail account and got a different message. Don't know if this is related or if is a whole different issue.
This is the mail system at host myhost.mydomain.com.
I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
<myemail@gmail.com>: host gmail-smtp-in.l.google.com[2607:f8b0:400e:c09::1a]
said: 550-5.7.1 [2605:f700:40:401::35c8:2c6f] Our system has detected that
this 550-5.7.1 message does not meet IPv6 sending guidelines regarding PTR
records 550-5.7.1 and authentication. Please review 550-5.7.1
https://support.google.com/mail/?p=IPv6AuthError for more information 550
5.7.1 . f6si1939294pgk.120 - gsmtp (in reply to end of DATA command)
Reporting-MTA: dns; myhost.mydomain.com.
X-Postfix-Queue-ID: 0D04991A2771
X-Postfix-Sender: rfc822; test@subdomain.mydomain.com
Arrival-Date: Mon, 10 Jun 2019 00:33:27 +0000 (UTC)
Final-Recipient: rfc822; myemail@gmail.com
Original-Recipient: rfc822;myemail@gmail.com
Action: failed
Status: 5.7.1
Remote-MTA: dns; gmail-smtp-in.l.google.com
Diagnostic-Code: smtp; 550-5.7.1 [2605:f700:40:401::35c8:2c6f] Our system has
detected that this 550-5.7.1 message does not meet IPv6 sending guidelines
regarding PTR records 550-5.7.1 and authentication. Please review 550-5.7.1
https://support.google.com/mail/?p=IPv6AuthError for more information 550
5.7.1 . f6si1939294pgk.120 - gsmtp
Subject Want it to work
From test@subdomain.mydomain.com
To myemail@gmail.com
Date Today 10:33