I have a website, and the users recommend via email to another persons, so the mail come from my site, but I need to appear the user as sender, but "via" mydomain. The logically and what lists management software and companies do is differ the "MAIL FROM" SMTP command and the "From" header (like mailchimp, for example).
So I want to DKIM sign a mail with mydomain.com, even if the From header says another domain. is possibly this?
I am using postfix with Debian GNU/Linux, and in this case using dkim-filter and dk-filter.