just someone want his privacy in his message, thats it!
Unless you have some real bad friends, why would you need to hide your ip in your e-mail. Am assuming you e-mail list consist of friends right! Oh BTW your ip is not a secret, if they want to get they are going to get it.

Am just wondering, what's the real reason why you want to do this? Otherwise from what you have already mentioned, I don't see why you need to do it...

Too much paranoia can drive you crazy....

Just my thought on things...