What does the SPF a Mechanism do?

`a` — SPF Mechanism

The SPF a mechanism authorises the IP address(es) returned by a domain's A or AAAA record. Learn when to use it and common pitfalls.

Type
Mechanism

Syntax
a[:<domain>][/prefix-length][//ipv6-prefix]

DNS Lookup
Yes — counts toward 10-lookup limit

Example
a:mail.example.com

💬 What This Mechanism Does

The "a" mechanism passes if the sending IP matches any A or AAAA record for the specified domain (defaulting to the sending domain if omitted). It's a convenient way to authorise a server you already use for your website or API without listing its IP directly.

When to Use This

Authorising a dedicated mail server whose IP matches its A record
Including a transactional server that shares your main domain's IP

⚠️ Watch Out For

Counts as one DNS lookup. If the domain has many A records, all are evaluated.
Avoid using bare "a" if your web server IP and mail server IP differ — it authorises the web IP too.