Global MX Record Checker
The MX (Mail Exchanger) record is the foundational routing protocol for incoming SMTP traffic. When an external Mail Transfer Agent (MTA)—such as Google Workspace or Microsoft Exchange—needs to deliver an email to user@example.com, it performs a DNS lookup against the example.com zone specifically requesting the MX records. The resulting payload provides a list of hostnames authorized to accept mail on behalf of that organization.
Priority Integers and SMTP Failover
Unlike standard A records, MX records implement a native hierarchy using priority integers (e.g., 10, 20, 50). This integer dictates the connection preference. The sending MTA will always attempt to establish a TCP handshake on port 25 with the server possessing the lowest numerical value. If that primary server refuses the connection, times out, or returns a temporary 4xx error, the MTA automatically falls back to the record with the next lowest number. Setting multiple MX records with the exact same priority integer enables rudimentary round-robin load balancing for inbound mail streams.
The Bare IP Violation (RFC 1035)
One of the most common configuration errors made by junior sysadmins is pointing an MX record directly to an IPv4 address. According to strict DNS standards (RFC 1035), the target of an MX record must be a canonical hostname, never an IP. For example, pointing an MX to 192.168.1.50 violates protocol. It must point to a hostname like mail.example.com, which then resolves to the IP via an A record. Strict corporate spam filters and intrusion detection systems will aggressively drop emails originating from or routing to domains with IP-based MX targets.
Null MX Records (RFC 7505) and Spam Mitigation
If a domain is used strictly for web traffic and is never intended to receive email, it is highly susceptible to backscatter spam. Attackers will spoof the domain in outbound spam, causing thousands of automated bounce messages to flood back to the domain. To mitigate this, administrators deploy a "Null MX" record. By configuring a single MX record with a priority of 0 and a target of . (the root), you mathematically instruct all global mail servers that the domain does not accept email, forcing them to drop the messages instantly without attempting delivery.