====== MX Records (Mail Exchanger) ======

A **Mail Exchanger (MX) Record** is a type of Resource Record in the Domain Name System (DNS) that specifies the mail server responsible for accepting email messages on behalf of a domain name.

Without valid MX records, sending servers cannot locate your mail infrastructure, causing incoming emails to bounce or fail delivery.

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===== How MX Records Work =====

When someone sends an email to ''user@example.com'', the sender's mail transfer agent (MTA) performs the following steps:

  # **DNS Lookup:** Queries DNS for the ''MX'' records associated with ''example.com''.
  # **Priority Resolution:** Sorts the returned MX hostnames by their preference (priority) number.
  # **A/AAAA Lookup:** Resolves the primary MX hostname to an IP address (''A'' or ''AAAA'' record).
  # **SMTP Connection:** Attempts an SMTP connection to the primary mail server on port ''25''. If the primary server is unreachable, it attempts to connect to secondary backup servers in order of priority.

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===== MX Record Structure & Parameters =====

An MX record consists of three core components:

^ Component ^ Value Example ^ Description ^
| **Name / Host** | ''@'' or ''example.com'' | The domain name receiving the email. |
| **Preference / Priority** | ''10'' | Integer value indicating server priority (lower numbers = higher priority). |
| **Mail Server / Target** | ''mail.example.com'' | The Fully Qualified Domain Name (FQDN) of the mail server. |

> **Crucial Rule:** The target of an MX record **MUST** point to a valid domain name (''A'' or ''AAAA'' record). It **must never** point directly to an IP address or a ''CNAME'' alias.

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===== MX Record Priority Explained =====

Priorities allow domain administrators to set up primary and backup fallback mail servers.

<code>
Priority 10 : mail1.example.com  <-- Primary Mail Server (Tried First)
Priority 20 : mail2.example.com  <-- Secondary / Backup Server (Tried if Priority 10 fails)
</code>

  * **Equal Priorities:** If multiple servers share the same priority (e.g., two servers with priority ''10''), incoming mail load is distributed between them (round-robin / load balancing).
  * **Lower Number = Higher Preference:** Priority ''5'' takes precedence over Priority ''10''.

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===== Example Configurations =====

==== Example 1: Standard Self-Hosted Mail Server ====

^ Record Type ^ Host ^ Priority ^ Value / Target ^ TTL ^
| **MX** | ''@'' | 10 | ''mail.example.com.'' | 3600 |
| **A** | ''mail'' | — | ''192.0.2.25'' | 3600 |

==== Example 2: Google Workspace Configuration ====

^ Record Type ^ Host ^ Priority ^ Value / Target ^ TTL ^
| **MX** | ''@'' | 1 | ''SMTP.GOOGLE.COM.'' | 3600 |

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===== Common Issues & Troubleshooting =====

==== 1. Pointing MX Records to CNAMEs ====
Pointing an MX record to a target that is a CNAME alias violates [[https://datatracker.ietf.org/doc/html/rfc2181|RFC 2181 (Section 10.3)]] and can lead to unpredictable mail loop errors or bounced emails.

==== 2. Pointing MX Records directly to IP Addresses ====
MX records require an FQDN as their target. Pointing directly to an IPv4/IPv6 address will cause syntax validation failures across major mail servers.

==== 3. Missing Trailing Dot (in Raw Zone Files) ====
In raw DNS zone files, failing to append a trailing dot (''mail.example.com.'') may cause DNS servers to append the root domain twice (''mail.example.com.example.com'').

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===== Verification & Testing =====

You can test and verify your MX records using **Telenegar Tools** or terminal commands:

==== Using Telenegar Tools ====
  * **DNS Resolution:** Check MX lookup propagation with the [[tools:dnscheck|Telenegar DNS Check Tool]].
  * **Mail Server Health:** Run a complete deliverability scan with the [[tools:mailcheck|Telenegar Mail Check Tool]].

==== Using Terminal Tools ====

=== dig (Linux / macOS) ===
<code> bash
dig +short example.com MX
</code>

=== nslookup (Windows / Cross-platform) ===
<code> cmd
nslookup -type=MX example.com
</code>

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===== Related Documentation =====

  * [[tools:dnscheck|Telenegar DNS Check Tool]] — Diagnostic tool for DNS record resolution and propagation.
  * [[tools:mailcheck|Telenegar Mail Check Tool]] — Diagnostic tool for email authentication and mail server health.
  * [[email:dns-configuration:spf|SPF (Sender Policy Framework)]] — Protect your domain against email spoofing.
  * [[email:dns-configuration:dkim|DKIM (DomainKeys Identified Mail)]] — Add cryptographic signatures to outbound emails.
  * [[email:dns-configuration:dmarc|DMARC]] — Define policy rules for SPF and DKIM alignment.
  * [[networking:dns:reverse-dns|Reverse DNS (PTR Records)]] — Map mail server IP addresses back to domain names.