Mechanisms can be used to describe the set of hosts which are designated outbound mailers for the domain and can be prefixed with one of four qualifiers:

+ (Pass)
​– (Fail)
​~ (SoftFail)
​? (Neutral)

If a mechanism results in a hit, its qualifier value is used. The default qualifier is “+ “, i.e., “Pass.” Mechanisms are evaluated in order. If no mechanism or modifier matches, the default result is “Neutral.”

Available Mechanisms:

Evaluation of an SPF record can return any of these results:

This mechanism always matches. It should always go at the end of the SPF record.

​Examples:

Allow domain’s MXs to send mail for the domain, prohibit all others.

The domain sends no mail at all.

The domain allows all IP addresses on the internet to send mail. Though ‘valid,’ this is not recommended.

The argument to the “ip4:” mechanism is an IPv4 network range. If no prefix-length is given, /32 is assumed (singling out an individual host address). Be careful to include a prefix-length greater than /16, as delivery to small, smaller receivers may be impacted.

​Example:

Allow any IP address between 192.168.0.1 and 192.168.255.255.

The argument to the “ip6:” mechanism is an IPv6 network range. If no prefix-length is given, /128 is assumed (singling out an individual host address).

​Examples:

Allow any IPv6 address between 1080::8:800:0000:0000 and 1080::8:800:FFFF:FFFF.

Allow any IPv6 address between 1080::8:800:0000:0000 and 1080::8:800:FFFF:FFFF.

All the A records for the domain are tested. If the client IP is found among them, this mechanism matches. If the connection is made over IPv6, then an AAAA lookup is performed instead.

If the domain is not specified, the current domain is used.

The A records have to match the client IP exactly unless a prefix-length is provided, in which case each IP address returned by the A lookup will be expanded to its corresponding CIDR prefix, and the client IP will be sought within that subnet.

​Examples:

The current domain is used.

Equivalent if the current domain is example.com.

Perhaps example.com has chosen to explicitly list all the outbound mailers in a special A record under mailers.example.com.

If example.com resolves to 192.0.2.1, the entire class C of 192.0.2.0/24 would be searched for the client IP. Similarly, for offsite.example.com. If more than one A record were returned, each one would be expanded to a CIDR subnet.

All the A records for all the MX records for the domain are tested in order of MX priority. If the client IP is found among them, this mechanism matches.

If the domain is not specified, the current domain is used.

The A records have to match the client IP exactly unless a prefix-length is provided, in which case each IP address returned by the A lookup will be expanded to its corresponding CIDR prefix, and the client IP will be sought within that subnet.

Examples:

Perhaps a domain sends mail through its MX servers plus another set of servers whose job is to retry mail for deferring domains.

Perhaps a domain’s MX servers receive mail on one IP address but send mail on a different but nearby IP address.

The hostname or hostnames for the client IP are looked up using PTR queries. The hostnames are then validated: at least one of the A records for a PTR hostname must match the original client IP. Invalid hostnames are discarded. If a valid hostname ends in the domain, this mechanism matches.

If the domain is not specified, the current domain is used.

If possible, you should avoid using this mechanism in your SPF record because it will result in many expensive DNS lookups.
​

Examples:

A domain that directly controls all its machines (unlike a dialup or broadband ISP) allows all its servers to send mail. For example, hotmail.com or paypal.com might do this.

Any server whose hostname ends in otherdomain.com is designated.

Perform an A query on the provided domain. If a result is found, this constitutes a match. It doesn’t matter what the lookup result is – it could be 127.0.0.2.

When you use macros with this mechanism, you can perform RBL-style reversed-IP lookups or set up per-user exceptions.

Example:

In the following example, the client IP is 1.2.3.4, and the current domain is example.com.
​

If example.com does not resolve, the result is failed. If it does resolve, this mechanism results in a match.

The specified domain is searched for a match. If the lookup does not return a match or an error, processing proceeds to the next directive.

Warning: If the domain does not have a valid SPF record, the result is a permanent error. Some mail receivers will reject based on a PermError.

​Example:

In the following example, the client IP is 1.2.3.4, and the current domain is example.com.

If example.com has no SPF record, the result is PermError.
Suppose example.com’s SPF record was “v=spf1 a ~all”.
Look up the A record for example.com. If it matches 1.2.3.4, return Pass.

If there is no match, other than the included domain’s “~all” they include as a whole fails to match; the eventual result is still Fail from the outer directive set in this example.

Trust relationships — The “include:” mechanism is meant to cross administrative boundaries. Great care is needed to ensure that “include:” mechanisms do not place domains at risk for giving SPF Pass results to messages resulting from cross-user forgery. Unless technical mechanisms are in place at the specified other domain to prevent cross-user forgery, “include:” mechanisms should give a Neutral rather than Pass result. This is done by adding “?” in front of “include:”.

Example:

Modifiers are optional. A modifier may appear only once per record. Unknown modifiers are ignored.

Available Modifiers:

The SPF record for the domain replaces the current record. The macro-expanded domain is also substituted for the current domain in those lookups.

If a ‘redirect’ modifier is used, the SPF record should not include the ‘all’ mechanism. If both are present, the ‘redirect’ modifier is ignored. Any ‘redirect’ modifiers beyond the first will be ignored.

Example:

In the following example, the client IP is 1.2.3.4, and the current domain is example.com.

If example.com has no SPF record, that is an error; the result is unknown.
Suppose example.com’s SPF record was “v=spf1 a ~all”.
Look up the A record for example.com. If it matches 1.2.3.4, return Pass.
If there is no match, the exec fails to match, and the ~all value is used.
​

If an SMTP receiver rejects a message, it can include an explanation. An SPF publisher can specify the explanation string that senders see. This way, an ISP can direct nonconforming users to a web page that provides further instructions about how to configure SASL.

The domain is expanded; a TXT lookup is performed. The result of the TXT query is then macro-expanded and shown to the sender. Other macros can be used to provide a customized explanation.

The exp modifier may only contain printable ASCII characters.

Over the past decade, it has become increasingly easier to send an email. Countless Sources have entered the marketplace, each providing a specialized toolset tailored to address marketers, developers, and small businesses' modern-day needs. Along with this expansion, email authentication, specifically SPF, has become an increasingly complex matter to navigate.

Within the SPF RFC specification (essentially internet law), there lies a practical limit of how many “DNS-querying mechanisms” a single SPF record can contain. That limit is ten. The ten maximum lookup states that a domain administrator (that’s you!) will not require the likes of Gmail or other receivers to conduct more than ten consecutive DNS lookups to see if you authorize a particular IP address to send mail on your behalf.

As it has become somewhat commonplace for any single organization to authorize a large number of disparate netblocks (due to the outsourced nature of email infrastructure), there remains what seems like the constant and unnecessary encroachment on the ten maximum lookup. However, this limit remains entirely practical and should be observed to ensure timely delivery and favorable inbox rates. Further, the solution to avoid the limit is squarely addressed by other mainstream emails best practices, long encouraged by major inbound receivers such as Gmail and Yahoo.

The single most practical solution to avoid the ‘too many lookups’ issue is to use sub-domains. As each discrete sub.domain is afforded its own ten lookup maximum, SPF is effectively boundless. Example: hello.com is permitted ten lookups + sub.hello.com is permitted ten lookups. Plainly put, you should never run into the ten maximum lookup condition if you are correctly segmenting different mail streams (e.g., transactional, corporate, marketing, etc.) on to discrete namespace.

In this subsection “delivery tips’ of the Gmail Postmaster Site, it is recommended to;