If someone wants you to do something and your answer is “yes,” put it in an email. This is for several reasons, many of which are naturally the opposite of why you don’t say “no” in an email.
First, use email as a tool to document your affirmative response. This then serves as a lasting record of your positive, and probably cooperative, reply. It also becomes their proof of purchase.
Second, people like to see “yes” in their inbox. It’s pretty much why people check their email 453 times per day. It’s like they received a gift or a prize or candy. It’s such a great feeling to ask something of someone and get a “let’s do it” or an “of course” or a “yes” back.
Third, email can be forwarded to 1 person and 1 million people. And in this case, that’s a really good thing because all of those people can see that you are the one that said “yes.” That you started something (or at least didn’t kill it) and that’s exactly the kind of thing you should want to be known for.
There are of course exceptions to this principle. The main one being, you may want to tell someone the good, cooperative, “can do” news in person. It may make you or, more importantly, the other person feel good. These are the times you want to deliver the good news face-to-face so that the other person can get a true sense of the emotional impact behind your answer.
Also, sometimes saying “yes” isn’t a good thing. “Yes, you have a rare skin rash,” or “yes, I was the one who wrecked your car.” Those are the kind of things best left to verbal, in-person communication.