email keeps getting blacklisted

I am working at a site that is a non-profit organization that sends out email blast to it's membership at least once a week. The size of the email list range from 500 - 15,000 depending on the intended audience. The emails contents are newsletters, magazine, safety classes, government regulatory changes, etc. None of the emails are spam and all have opt out options. I checked the reverse DNS and that is ok. It seems that the problem is the volume.

I was hoping to find a whilelist service that has pricingb that is reasonable for a non-profit. I just cannot seem to find a solution.

Has anyone had this issue, how did youn resolve it. Does anyone know of a service?