In mail log we can see mail arrive...
Jun 12 04:29:01 SERVERNAME postfix/smtpd[3985]: NOQUEUE: milter-reject: MAIL from de-smtp-delivery-223.mimecast.com[194.104.111.223]: 451 4.3.5 Server configuration problem - try again later; from=<> proto=ESMTP helo=<de-smtp-delivery-223.mimecast.com>
Jun 12 04:29:01 SERVERNAME postfix/smtpd[3985]: disconnect from de-smtp-delivery-223.mimecast.com[194.104.111.223] ehlo=1 starttls=0/1 mail=0/1 quit=1 commands=2/4
Jun 12 04:29:27 SERVERNAME postfix/smtpd[3985]: connect from sw.managed-otrs.com[159.69.61.96]
Jun 12 04:29:27 SERVERNAME postfix/smtpd[3985]: warning: milter inet:127.0.0.1:12768: bad protocol information: socks
Jun 12 04:29:27 SERVERNAME postfix/smtpd[3985]: NOQUEUE: milter-reject: CONNECT from AlfredKeller.managed-otrs.com[159.69.61.96]: 451 4.3.5 Server configuration problem - try again later; proto=SMTP
Jun 12 04:29:27 SERVERNAME postfix/smtpd[3985]: NOQUEUE: milter-reject: MAIL from AlfredKeller.managed-otrs.com[159.69.61.96]: 451 4.3.5 Server configuration problem - try again later; from=< > proto=ESMTP helo=<sw.managed-otrs.com>
it is something wrong in PLESK
but as we use the migration tool, we should have the same configurations