Fuck you fuckin spammer! Because of the arms race it started that getting those message some days ago:
Undelivered Mail Returned to Sender
gmail-smtp-in.l.google.com[2a00:1450:4013:c00::1a] said: 550-5.7.1 [2a01:488:66:1000:5bfa:7407:0:1] 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=ipv6_authentication_error for more 550 5.7.1 information. m131si2394453wmb.65 - gsmtp (in reply to end of DATA command)
TL;DR MSMTP is a beyond edge of plate tip when I ran into the DKIM issue. Nothing what follows is really new or surprising.