I can understand his wish for formality. I need to be very formal at times, and at time I get criticized for being too formal. If he didn't want to be addressed informally he should have found a way to sign up in a more formal way.
If he is ticked because it is a mass e-mail .... Well ... EXCUUUUUUUUUUUUUUUUUUUSE ME !!! Does he honestly expect you to sit and had address each and every e-mail in a custom way?
I believe you need to listen to each and every comment you receive, signed or not. However, you need to weigh them for importance. If this is your first complaint in many years it needs to be filed away as Bad - 1, Good - X, No Response - Millions. Watch for a trend, but so far he is only one that is representing a handful. I would guess that each of us that is supportive of your style also represent a handful ... You're doin' goods have it so far!