During my time as a sysop and later as CTO I had quite a few e-mail servers under me. Over 50 to be exact. These servers were not standalone ones, but passed on e-mails to each other. We designed the system to avoid bottlenecks and make it easily extendable. It fully met the expectations in this respect, it was very easy to plug additional nodes into the system.
There was however an other aspect, where the system was not so great. And as a matter of fact, I haven’t seen any e-mail system, that was great at this. What I mean is tracking e-mails, debugging problems. Having to SSH into just a fraction of this many servers or reading logs from this amount of servers is a really, really painful way to do it.