This is interesting because you're saying recipients are opening the mail and then choosing whether or not to reply after they read it. This shows e-mails are not just sitting unread.

I always figured that it was the subject line that mattered most and that once a mail was opened it was more likely they would reply.

I've experimented with improving e-mail structure. I always get a more detailed reply if I group any questions I want answering together as the final part of the mail.