Q. If a person sent you an NDA [Non-Disclosure Agreement], in the body of an email, and you were asked to email back "I agree" as the sign that you read and understood the particulars.... what do you think of the validity of that exchange?
Legally, I believe, even an oral agreement is binding. An email agreement is an agreement.
However, an email is just a text file. Any text file can be edited in a text editor. What happens if one of you alters your copy of the agreement? Then it's "he said, she said."
That's why Adobe PDF software enables cryptographic signatures on documents. An encrypted "signature" at least prevents tampering with the document.
In real life, people rarely forge documents. On the other hand, you keep reading about people who do. I'd stick with actual signatures.
(I am not a lawyer. This is not legal advice.)