From the mid-90s, struggling to find ways to load-balance incoming email, with DNS responses needing to fit into a UDP packet.
Thus, a.mx.aol.com was born. And so was b.mx.aol.com, c.mx.aol.com, and so on. By the time of the disaster, we had listed nine names in the DNS as our MXes, with five IP addresses each. All 45 of our mail servers could be listed in the DNS as MXes for AOL and would still fit into a 512-byte UDP response packet – just barely.