Nah it doesn't. Cache the list of emails that have unsubscribed, your dispatching process will filter out from that list.

Let's say 10million have unsubscribe, at 128bytes per email. That's roughly under 1.5 gigs of ram to store all those in memory.

Sending 2million emails daily comes out to an average of 23 a second. You can search 23 times through that list pretty quickly with one extra CPU core.

Pretty sure you could check a email in a database more than 23 times per second these days.

If it was sane company, you could. When I was doing some support for less sane companies, sometimes their "servers" were less powerful than my phone (hey, it still works after 10yrs, why change it). When my current company made product, everyone in such less sane companies was amazed that we could produce monthly route reports for fleet cars in seconds instead of minutes (and as devs we thought it was still not optimized enough).