It's not really the same... You are talking about hosting big mailboxes; the article talks about sending and receiving e-mail. Sent e-mails are probably just discarded and incoming mails go into a processing pipeline, which is handled externally, which removes the need for a web interface to browse messages, search them and maintain backups.
For reference Back in 1984 Huge was a cluster of 16 or so prime 750 super mini's over two Data centres - this was Telecom Gold.
Wish I had saved some of the printouts of the billing monitor when it was running (the monthly map reduce billing system) as it listed the DASD and memory for each member of the cluster.
> The system’s back end Cyrus message store comprises 16 single-CPU PCs, each with 3GB RAM and 7 72GB SCSI disks providing 350GB of available storage after RAID and filesystem overheads. The front end redirector ppsw is 6 dual-CPU PCs each with 1GB RAM and 4 18GB SCSI disks providing 50GB of available storage. The webmail system is another PC with the same configuration. The terminal-based Menu System is still running on the old Hermes systems at the moment, until migration to the new system is complete. It will then move onto a PC with plenty of RAM and CPU. The backup system is attached to a disk shelf containing 16 250GB SATA disks providing 3.2TB of usable storage, and a tape robot containing a 17 cartridge magazine and two LTO drives
These days each of those would be a "small" VPS.
Edit: just found an even better example, https://github.com/Exim/exim/wiki/Q1002 "On a PII 400 with 128M of RAM running Linux 2.2.5, I have achieved 36656 messages per hour (outgoing unique messages and recipients)"