MX Lookup

Mail Exchange (MX) records are defined in RFC 5321 and RFC 1035 as DNS resource records that specify the mail servers responsible for accepting email messages for a domain. Each MX record contains two fields: a priority value (16-bit unsigned integer, lower = higher priority) and a mail server hostname. When a sending mail server needs to deliver email, it performs a DNS MX lookup for the recipient domain, sorts results by priority, and attempts SMTP connection (TCP port 25) to each server in order. Multiple MX records provide redundancy: if the primary server (priority 10) is unreachable, delivery falls back to the secondary (priority 20). MX records must point to A/AAAA records — never directly to IP addresses or CNAME chains.

Properly configured MX records are a prerequisite for DKIM signing, SPF validation, and DMARC policy enforcement. The SMTP protocol (RFC 5321) defines the full delivery handshake: EHLO/HELO, MAIL FROM, RCPT TO, DATA, and QUIT. Mail servers may support STARTTLS (opportunistic TLS upgrade, RFC 3207) or use port 465 (SMTPS) for implicit TLS. Modern email security requires that MX records point to hosts with valid PTR (reverse DNS) records, properly configured TLS certificates, and SMTP banner hostnames that match the reverse DNS — violations can trigger spam filters.

Common MX configuration patterns include: Google Workspace (aspmx.l.google.com with priorities 1, 5, 10, 10, 10), Microsoft 365 (domain-com.mail.protection.outlook.com at priority 0), and self-hosted Postfix/Exim/Sendmail. Misconfigured MX records are one of the top causes of email delivery failures — organizations may accidentally point to decommissioned servers, have missing or expired PTR records, or have firewall rules blocking port 25 inbound. Regular MX health checks help proactively catch these issues before they result in bounced or undelivered email.

When diagnosing email delivery issues, MX record verification is always the first step. Administrators should confirm: (1) all MX hostnames resolve to valid A/AAAA records, (2) SMTP port 25 is reachable on each MX host, (3) reverse DNS (PTR records) match the forward hostname, (4) TLS certificates on MX hosts are valid and not expired. Tools like mxtoolbox.com, dmarcanalyzer.com, and this tool provide automated verification. Email providers like Gmail and Outlook increasingly reject mail from senders with DNS misconfigurations, making regular MX audits essential for maintaining email deliverability.