Sometimes the way you separate addresses makes a difference. It usually likes a ; between each address. If you open your address book and select all of the names at once (you may have to hold the control key down to do this) then click to send to each of them they should by default have the proper punctuation between names.

I hope you're using BCC--Blind Carbon Copy--when you send to a bunch of people. Then recipients don't see everyone else's email address. When you use that it is often best to mail the post to yourself and put the list in BCC.

I hope this helps.

SRS