The newsletter is only set to people who have completed the double opt in process.
It isn't clear from your post but I am assuming that you mean 16 newsletters not 16 opt in requests. That sounds as if only one person from the 55 had opted in by the time the newsletter was sent.
Opt in requests have a high attrition rate and are often blocked by spam filters. If these are addresses that you have loaded yourself (again trying to read between the lines of your post) I think you will be lucky to get acceptance in double figures even if they have expressly asked you to be on the mailing list.