/etc/dkimproxy/dkimproxy_out.conf:
listen    127.0.0.1:10027
relay     127.0.0.1:10028
domain
keyfile   /var/lib/dkimproxy/private.key
selector  default
sender_map /etc/dkimproxy/sender_map


/etc/dkimproxy/sender_map:
domain1.com  dkim(a=rsa-sha256,c=relaxed,key=/var/lib/dkimproxy/domain1-dkim.key),             domainkeys(a=rsa-sha1,c=nofws,key=/var/lib/dkimproxy/domain1-dk.key)
domain2.com  dkim(a=rsa-sha256,c=relaxed,key=/var/lib/dkimproxy/domain2-dkim.key),             domainkeys(a=rsa-sha1,c=nofws,key=/var/lib/dkimproxy/domain2-dk.key)


/etc/postfix/main.cf:
content_filter = dkimsign:[127.0.0.1]:10027


/etc/postfix/master.cf:
dkimsign    unix  -       -       n       -       2      smtp
    -o smtp_send_xforward_command=yes
    -o smtp_discard_ehlo_keywords=8bitmime,starttls

127.0.0.1:10028 inet  n  -      n       -       10      smtpd
    -o content_filter=
    -o receive_override_options=no_unknown_recipient_checks
    -o smtpd_helo_restrictions=
    -o smtpd_client_restrictions=
    -o smtpd_sender_restrictions=
    -o smtpd_recipient_restrictions=permit_mynetworks,reject
    -o mynetworks=127.0.0.0/8
    -o smtpd_authorized_xforward_hosts=127.0.0.0/8