I actually cryptographically sign my emails* using an s/mime certificate, and my mail provider uses DKIM + SPF (gmail).
If you want real, unfalsifiable signatures, there is absolutely nothing else except strong cryptography. There never will be (In addition, you can do the same thing to PDFs using the same certificate).
I use the entire document, hashed, and then encrypt it with my private key and append my public key to documents (this ensures that the message was sent by me and has not been tampered with).
But I'm a web developer, systems administrator, and systems penetration tester, so I guess my clients are actually way more likely to even begin to understand what a digital signature even is.
If you want to use email as a signature or contract agreement, I strongly recommend that you cryptographically sign them. If this is not possible, then save the entire email, which will most likely have more headers than actual text. This is possible with almost all email providers.
The email will look a little something like:
Content-Type: multipart/mixed; charset=utf-8; boundary=---EMAIL---
And so on. This is not perfect, and can still be spoofed, but is far better than a copy-pasted word document of an email (in fact, that is not actually an email1; a screenshot of your email program would do better)
* I have actually signed my housing tenant agreement using a 4096bit RSA key. It was accepted, to my astonishment.
1: I'm not a lawyer.