Made me look ...
The canonical answer is: If you look at /etc/rc.subr, you see that you can use "NO" in any mixture of upper and lower case (so "nO" and "No" would be valid, but ugly and a bad idea), as well as "false" and "off" (again in any case). And for "yes", you can also use "true" and "on".
In the case of sendmail, please look at /etc/rc.d/sendmail. Right at the top, if you set the variable sendmail_enable to "none" (in lower or upper case), then it gets automatically changed to "no". So it seems that you are not the first person to ask this question, and because others have gotten it wrong before, "none" is equivalent to "no".
So the formal answer to your question is: Both are valid. That immediately leads to the next question: Which one should you use? And should you use no, NO, NoNe, nOnE, fAlSe or oFf, or some other perverse mix? Here my advice would be: It seems that the distributed defaults all use YES and NO (in all uppercase), so best to stick to that. You know, when in Rome, do like the Romans ...