Measuring Basic Load-Balancing and Fail-Over Setups for Email Delivery via DNS MX Records

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.