Drip for very large systems

I have used www.drip.com successfully in the past and it works well when I getting a dozen or so signups per day. However I am looking at introducing a drip email system for a product that gets 5000 signups PER DAY. This is way above any plans Drip has advertised. Are there any drip systems that cater to this load (for a reasonable price) or am I just best on building it myself?