I have set up SPF and DKIM for the domains provided by Froxlor, that works pretty straight forward except for the opendkim thing, but I got it working.
Now I am wondering if it is possible to add a DNS record for DMARC to each domain that is maintained automatically, this would be a TXT record
with _dmarc TXT "fixed string" or a domain-by-domain entry when using a different feedback address for different domains
for domains that are manually maintained that works by adding it manually, but a generic rule for all domains would be helpful