How is there not an "SSL padlock" type feature in email clients based on, say, the DKIM signature? Sure it won't tell you that the actual person sent it, just

How is there not an "SSL padlock" type feature in email clients based on, say, the DKIM signature? Sure it won't tell you that the actual person sent it, just that it came from a particular server. But it's nearly impossible at that point to even pretend to be anything@whitehouse.gov if you're missing the DKIM sig.