IMHO forged email headers are a minor issue.
Do you really look at email headers before deciding to run an executable attached to an email? If Hallmark really did send you an .exe you still should't trust it I hope - so isn't the executable attachment a far bigger deal than the forged header line(s)?
BTW, I could send you a postcard on which I'd written "From: Barack Obama, 1600 Pennsylvania Avenue" but in addition to that there'd like be a postmark saying "Innsbruck, Austria" [I'm doing a bit of skiing this week].
Would you believe my forged "From:" lines, or the post office's postmark?
Just like "From:" and "Received:" in your email header... it's easy once you know how :-)