You are begging the question by assuming that a signature on an email message proves anything useful to the recipient when the facts on the ground show this not to be the case.
It is not harder to conduct phishing with email signatures, and the fact that such phishing campaigns have no problem putting TLS certs on their phishing sites is a simple existence proof of this fact.
Email signatures does not impact spam in any significant manner beyond existing measures to prevent domain name forgery in the header. Spam email signed by joe@cheap-ray-bans.com does not stop being spam because it is signed and the signature provides no useful signal to spam filtering tools.
The difference between an email signature and a TLS cert on a web site is that in the latter case the user is making an effort to connect to a specific site and the certificate ensures that they are in fact connecting to paypaaaaal.com even if other means were used to misdirect them to this site. With email there are two problems to be addressed, transport privacy/security (a sender problem) and unsolicited email (a recipient problem) and signatures are only useful in ensuring integrity of the former and do nothing for the latter.
It is not harder to conduct phishing with email signatures, and the fact that such phishing campaigns have no problem putting TLS certs on their phishing sites is a simple existence proof of this fact.
Email signatures does not impact spam in any significant manner beyond existing measures to prevent domain name forgery in the header. Spam email signed by joe@cheap-ray-bans.com does not stop being spam because it is signed and the signature provides no useful signal to spam filtering tools.
The difference between an email signature and a TLS cert on a web site is that in the latter case the user is making an effort to connect to a specific site and the certificate ensures that they are in fact connecting to paypaaaaal.com even if other means were used to misdirect them to this site. With email there are two problems to be addressed, transport privacy/security (a sender problem) and unsolicited email (a recipient problem) and signatures are only useful in ensuring integrity of the former and do nothing for the latter.