Wietse Venema schreef op 21-10-2019 18:10:

> You can remove all MIME-Version headers with the header_checks
> IGNORE action (header_checks has no counter). But that will
> almost certainly break signed messages (DKIM/DMARC).

Thank you. The clients are quite outdated (which is why they don't use 
SES directly) and they don't sign their messages with DKIM or with 
DMARC. So now I remove all MIME-Version headers and the mail is accepted 
by SES.
And there was much rejoice.

Now I will try to convince the senders to fix their application.

Regards,
Lucas