Need specialist help for our clients mails being flagged as spam by Gmail and Hotmail/Outlook

Only serious candidates with experience preventing legitimate mails being sent as spam. We are looking for a anti-spam specialist for a client of ours. Our client is sending 10-15k emails for different of his clients on a monthly basis (around 200k mails per month) The mails are sent from a single server and we have changed the IP-address for them as a test, but GMAIL and Outlook keep flagging the mails for all of his clients as spam (typical GMAIL message: Why is this message in Spam? It contains content that's typically used in spam messages.) We are looking for someone to help us, by doing an analysis of the current situation, pointing out the problems and then presenting steps towards a solution. Before the project is awarded, we require a small analysis to point out the problems , based on the contents of this analysis , we will award the project. The initial project price, can be an initial budget. We can discuss the further steps.