Infrastructure

A Record

Definition

An A record (“address record”) is the DNS entry that maps a hostname to an IPv4 address. For example, it points mail.example.com to 203.0.113.10. It is one of the most fundamental DNS record types, defined in RFC 1035, and in email it is what lets a mail server’s hostname resolve to an actual IP that other servers can connect to.

  • Maps a hostname to an IPv4 address
  • Without it, an MX hostname cannot resolve to an IP
  • AAAA is the same idea for IPv6 addresses
  • A missing or mismatched A record can land mail in spam
At a glance
Record type DNS A (Address)
Maps Hostname to IPv4 address
IPv6 equivalent AAAA record
Defined in RFC 1035 (1987)
Email role Resolving mail-server & HELO hostnames

What an A record does

DNS is the internet’s address book, and the A record is its most basic entry: it answers the question “what IPv4 address does this hostname point to?” When something looks up mail.example.com and gets back 203.0.113.10, that answer came from an A record. The IPv6 version of the same record is the AAAA record (four times the address length, hence the four A’s).

A standard A record: hostname, TTL in seconds, class, type, and the IPv4 address 
mail.example.com.   3600   IN   A   203.0.113.10

Why A records matter for email

Email leans on A records at several points. An MX record points to the hostname of a mail server, and that hostname needs an A (or AAAA) record so the sending server can resolve it to an IP and connect. Per the DNS specs an MX record must point to such a hostname, never to a CNAME alias. The hostname a server announces in its HELO/EHLO greeting should likewise resolve to an A record.

For deliverability, the key link is between forward and reverse DNS. Many receivers check that your sending IP’s PTR record resolves to a hostname, and that the hostname’s A record resolves back to the same IP, a match known as forward-confirmed reverse DNS. If the A record is missing or doesn’t match, your mail is far more likely to be rejected or filtered to spam.

Where the A record fits in mail delivery

A sending server has mail for you@example.com
It looks up the MX record for example.com
The MX points to a mail-server hostname
The hostname’s A record resolves to an IPv4 address
The server connects to that IP and delivers

A vs AAAA vs CNAME

A AAAA CNAME
Maps to IPv4 address IPv6 address Another hostname
Example 203.0.113.10 2001:db8::10 mail.host.net
Valid MX target? Yes Yes No (breaks RFC 2181)

By the numbers

32-bit
The size of the IPv4 address an A record points to; the AAAA record points to a 128-bit IPv6 address.
FCrDNS
Forward-confirmed reverse DNS, the A-plus-PTR match many receivers now expect before accepting mail.

Common mistakes

Pointing an MX record at a CNAME
An MX record must reference a hostname that has its own A/AAAA record. Aiming it at a CNAME alias violates the DNS specs and breaks mail on strict receivers.
Forward and reverse DNS that don’t match
If your IP’s PTR points to a hostname whose A record resolves to a different IP, forward-confirmed reverse DNS fails, a common and quiet cause of spam-foldering.

Frequently asked questions

Do I need an A record for email?
Indirectly, yes. Your MX record points to a mail-server hostname, and that hostname needs an A (or AAAA) record to resolve to an IP. Without it, other servers can’t connect to deliver mail. The sending IP’s hostname should also have a matching A record for forward-confirmed reverse DNS.
Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026 ← Back to glossary