Thanks for the quick and honest answers :)
About DKIM: It adds another layer of authentication to the email by adding a signature. It being absent isn't really a bad indicator, because unfortunately email headers might change during delivery. This will invalidate the DKIM signature. But it being there is a strong positive that the email comes from the domain it says it does. From another perspective: An email that might be filtered as spam without DKIM is more likely to go through with a positive DKIM result.