Whitelisting a IP/domain to send 1000 emails per day from a website

Job to do described in a single sentence: Whitelisting IP/ domain name to be able to send hundreds or even thousands of emails per day from a website to gnail, hotmail, etc emails, and not to get it blacklisted again. My website has a busy active forums and blogs. Over hundreds of messages per day. Over 1000s of people following messages and expecting to get notifications on new messages and new replies. Up to 2000 emails to be sent per day. Before the first email is sent, they will receive an email asking them to confirm their email address and to confirm that they want to receive email notifications. That means server needs to send hundreds of emails per day, even thousands. How to do it the right way so my domain / ip address are not blacklisted by big email providers like gmail. Over 90% of my users use gmail. Need help with setting up proper domain / network / email settings so that gmail, hotmail, etc are not blocking emails. Help from someone who have done it before and knows exactly how to do it so it works in the future. It is not a big job, but special skills and special type of experience is needed. No spam sending in any form. The website has been online for 16 years and have over 200.000 registered members. People have possibility to follow the discussions and receive an email every time a new message is posted in any forum, or on any blog. They do not get email automatically by signing on the website, but have to opt to receive it. They can opt out from receiving any emails by selecting a single checkbox on the user account.