  The domain name system (DNS) has long provided means to assure basic
load-balancing and fail-over (BLBFO) for email delivery. A traditional method
uses multiple mail exchanger (MX) records to distribute the load across
multiple email servers. Round-robin DNS is the common alternative to this
MX-based balancing. Despite the classical nature of these two solutions,
neither one has received particular attention in Internet measurement research.
To patch this gap, this paper examines BLBFO configurations with an active
measurement study covering over 2.7 million domains from which about 2.1
million have MX records. Of these MX-enabled domains, about 60% are observed to
use BLBFO, and MX-based balancing seems more common than round-robin DNS. Email
hosting services offer one explanation for this adoption rate. Many domains
seem to also prefer fine-tuned configurations instead of relying on
randomization assumptions. Furthermore, about 27% of the domains have at least
one exchanger with a valid IPv6 address. Finally, some misconfigurations and
related oddities are visible.
