I've heard many sites are having issues with Gmail unfortunately. I think it is very important for sites to not keep sending to people who don't open or click mails... otherwise it looks a lot like spam if there is no interaction. At my sites we automatically stop sending mails to users who never read, so you are always sending to an active list. Sure, looks nice to be able to say "Mail to 100,000", but when that brings in only a few clicks that is just a marketing spin and not actual value.