Trust Model for e-mail

No, self-signed certificate not much better
than the From: header address
Sender creates both
Either might be fake

But
Certificate can be widely published
Claimed to belong to me
If fake
hope someone will notice
expose the forgery
Better than nothing

Better still
If I know you
You tell me that is your certificate
I can sign it for you
You tell other friends
They also sign your certificate