How MX Records Work: A Developer's Guide to Receiving Email

When developers think about DNS, they often think about connecting a domain to a website.

But DNS also plays an important role in email delivery.

If a domain needs to receive email, one of the most important DNS records to understand is the MX record.

MX stands for Mail Exchange.

In this post, I'll explain what MX records are, how mail servers use them, how priority works, what can go wrong, and why MX records are important when building email-based applications.

What Is an MX Record?

An MX record is a type of DNS record that tells mail servers where email for a particular domain should be delivered.

For example, suppose someone sends an email to:

[e-mail]

The receiving domain is:

example.com

The sending mail server needs to find out which mail server is responsible for receiving email for example.com.

That's where the MX record comes in.

A simplified example might look like:

example.com → mail.example.com

The sending system can then resolve mail.example.com to an IP address and connect to the appropriate mail server.

Why Can't Email Just Use the A Record?

This is a common question when learning DNS.

An A record maps a domain or hostname to an IPv4 address.

For example:

example.com → 192.0.2.10

An MX record has a different purpose.

It specifically tells mail systems which host is responsible for receiving email for a domain.

For example:

example.com → mail.example.com

The mail server hostname can then have its own DNS record that resolves to an IP address.

This separation allows a domain's website and email infrastructure to use different servers.

For example:

example.com → website server

while:

example.com → MX → mail.example.com

The website and mail infrastructure don't have to run on the same server.

How Email Uses an MX Record

Let's say I send an email to:

[e-mail]

The sending mail server needs to determine where example.com accepts email.

It performs a DNS lookup for the domain's MX records.

Suppose the domain returns:

10 mail.example.com

The number 10 is the MX priority.

The sending server then resolves mail.example.com to an IP address and attempts to connect to the mail server.

If the receiving server accepts the message, the email can continue through the receiving system for processing and delivery.

So, at a high level:

Email address → Domain → MX lookup → Mail server → SMTP connection → Message processing

Understanding MX Priority

A domain can have multiple MX records.

For example, a domain might have:

10 mail1.example.com

20 mail2.example.com

The number represents the priority.

With MX records, a lower number means higher priority.

So the mail server with priority 10 is preferred over the server with priority 20.

If the preferred server cannot accept the connection, the sending system may try another available MX server according to the domain's configuration.

This provides a way to configure multiple receiving mail servers.

Why Use Multiple MX Records?

Multiple MX records can provide redundancy.

Imagine a domain has only one receiving mail server.

If that server becomes unavailable, incoming email delivery can be affected.

With multiple MX records, the domain can specify additional mail servers.

For example:

10 mail1.example.com

20 mail2.example.com

If the first server isn't available, the sending system can attempt another configured server.

The exact behavior depends on the sending system and the reason for the failure, but the general purpose is to provide additional delivery options.

What Happens If a Domain Has No MX Record?

This is where things can become interesting.

If a domain doesn't have an MX record, mail systems may fall back to other DNS information according to the applicable email standards and implementation.

In many common configurations, an A or AAAA record for the domain may be used as a fallback destination.

However, relying on this behavior isn't a substitute for correctly configuring MX records.

If a domain is intended to receive email, explicitly configuring the appropriate MX records is the clearer and more reliable approach.

How to Check MX Records

Developers don't need a special email application to inspect MX records.

You can use DNS tools to check them.

For example, from a terminal, tools such as dig can be used:

dig MX example.com

You can also use other DNS lookup tools to inspect MX records.

The result can tell you which mail servers are configured for the domain and what their priorities are.

This is useful when debugging email delivery problems.

For example, if someone says:

"My domain isn't receiving email."

One of the first things worth checking is whether the domain has the expected MX records.

Common MX Configuration Problems

Incorrect MX configuration can cause email delivery problems.

Some common issues include:

1. Missing MX records

The domain may not have an MX record configured at all.

2. Incorrect mail server hostname

The MX record might point to a hostname that doesn't exist or doesn't resolve correctly.

3. Incorrect priority

Multiple MX records may be configured with unexpected priorities.

Remember that a lower number represents higher priority.

4. DNS changes haven't propagated everywhere

DNS changes can take time to become visible depending on caching and TTL settings.

This can make troubleshooting confusing because one DNS resolver may show the new configuration while another still has cached information.

5. The receiving server isn't accepting mail

Having a correct MX record doesn't guarantee successful email delivery.

The mail server itself must be available and correctly configured to accept incoming messages.

6. Firewall or network problems

The mail server may be correctly configured but unreachable because of network or firewall restrictions.

7. Recipient handling problems

The receiving server may be reachable but reject the message because the recipient doesn't exist or because another policy prevents delivery.

This is why debugging email requires looking at the entire delivery chain rather than checking only DNS.

MX Records and Temporary Email Systems

MX records become particularly important when building a temporary email system.

Suppose a temporary email platform uses a domain such as:

temporary-domain.com

Users might receive temporary addresses such as:

[e-mail]

For those addresses to receive email, the domain needs email-receiving infrastructure.

The MX record points incoming email toward the appropriate mail server.

The receiving system can then determine which temporary address the message belongs to and pass the message into the application's processing pipeline.

The application may then:

• Parse the message
• Identify the recipient
• Store message metadata
• Extract the sender and subject
• Detect OTP codes
• Process HTML content
• Handle attachments
• Apply expiration rules
• Delete messages automatically

This is one of the areas I've been working with while building InboxOro, where email infrastructure and application-level email processing have to work together.

MX Records Are Only One Part of Email Delivery

It's important to remember that an MX record doesn't handle the entire email-delivery process.

It primarily tells sending mail systems where to attempt delivery.

After that, many other things can happen.

The receiving server may perform:

• SMTP checks
• Recipient validation
• SPF checks
• DKIM verification
• DMARC evaluation
• Spam filtering
• Security checks
• Message processing
• Storage

So having a correct MX record is necessary for many receiving configurations, but it doesn't guarantee that every email will reach the inbox.

A Practical Debugging Approach

When troubleshooting an email-receiving problem, I like to break the problem into smaller parts.

Step 1: Check the domain

Is the domain resolving correctly?

Step 2: Check the MX record

Does the domain have the expected MX record?

Step 3: Check the MX hostname

Does the hostname specified by the MX record resolve correctly?

Step 4: Check mail-server connectivity

Can a sending mail server reach the receiving mail server?

Step 5: Check SMTP behavior

Does the receiving server accept the SMTP connection and recipient?

Step 6: Check application processing

If the mail server accepts the message, is the application processing and storing it correctly?

This approach helps narrow down where the actual problem is.

Instead of simply asking:

"Why didn't the email arrive?"

you can ask:

"At which stage did delivery fail?"

That is a much more useful question.

Conclusion

MX records are one of the fundamental pieces of email infrastructure.

They tell sending mail systems which servers are responsible for receiving email for a domain.

Understanding MX records helps developers troubleshoot problems involving:

• Email delivery
• Domain configuration
• Temporary inboxes
• Verification emails
• SMTP
• Mail-server infrastructure
• Email-based applications

And once you understand MX records, the next logical step is understanding what happens after a sending server connects to the receiving server.

That's where SMTP comes in.
I'm continuing to explore email infrastructure while building InboxOro, a temporary email and email infrastructure platform. I'll share more about SMTP, email processing, APIs, and developer tooling in future posts.

What email or DNS problem have you encountered while building an application?