[luckylion]

A lot of the spam these days is being sent via hacked computers. You'd get signed emails from individuals, only those individuals wouldn't know they are sending them.

It's the same problem with DDOS. Some providers tried to mail out letters "hey, your computer is involved in malicious things, please get it fixed", but that just lead to a lot more support requests that the ISPs can't handle. So we just accept that botnets are a thing.

[upofadown]

It is unlikely you would know the people that had the hacked computers. So that case would be the same as no signature at all. Otherwise the spammers could just make up their own valid signatures.