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”.
If a domain has no SPF record at all, the result is “None”. If a domain has a temporary error during DNS processing, you get the result “TempError” (called “error” in earlier drafts). If a syntax or evaluation error occurs (eg. the domain specifies an unrecognized mechanism) the result is “PermError” (formerly “unknown”).
v=spf1 -all
v=spf1 a -all
v=spf1 a mx -all
v=spf1 +a +mx -all
| Result | Explanation | Intended Action |
|---|---|---|
| Pass | The SPF record designates the host to be allowed to send | Accept |
| Fail | The SPF record has designated the host as NOT being allowed to send | Reject |
| SoftFail | The SPF record has designated the host as NOT being allowed to send but is in transition | Accept but mark |
| Neutral | The SPF record specifies explicitly that nothing can be said about validity | Accept |
| None | The domain does not have an SPF record or the SPF record does not evaluate to a result | Accept |
| PermError | A permanent error has occurred (eg. badly formatted SPF record) | Unspecified |
| TempError | A transient error has occurred | Reject |
This mechanism always matches. It should always go at the end of the SPF record.
v=spf1 mx ~all
Allow domain’s MXs to send mail for the domain, prohibit all others.
v=SPF1 -all
The domain sends no mail at all.
v=spf1 +all
The domain allows all IP addresses on the internet to send mail. This is a highly discouraged approach.
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 some smaller receivers may be impacted.
v=spf1 ip4:192.168.0.1/16 ~all
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).
v=spf1 ip6:1080::8:800:200C:417A/96 ~all
Allow any IPv6 address between 1080::8:800:0000:0000 and 1080::8:800:FFFF:FFFF.
v=spf1 ip6:1080::8:800:68.0.3.1/96 ~all
Allow any IPv6 address between 1080::8:800:0000:0000 and 1080::8:800:FFFF:FFFF.
All the A records for 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 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.
v=spf1 a ~all
The current domain is used.
v=spf1 a:example.com ~all
Equivalent if the current domain is example.com.
v=spf1 a:mailers.example.com ~all
Perhaps example.com has chosen to explicitly list all the outbound mailers in a special A record under mailers.example.com.
v=spf1 a/24 a:offsite.example.com/24 ~all
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 domain are tested in order of MX priority. If the client IP is found among them, this mechanism matches.
If 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.
v=spf1 mx mx:deferrals.domain.com ~all
Perhaps a domain sends mail through its MX servers plus another set of servers whose job is to retry mail for deferring domains.
v=spf1 mx/24 mx:offsite.domain.com/24 ~all
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 domain, this mechanism matches.
If 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 a larger number of expensive DNS lookups.
v=spf1 ptr ~all
A domain which 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.
v=spf1 ptr:otherdomain.com ~all
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.
In the following example, the client IP is 1.2.3.4 and the current domain is example.com.
v=spf1 exists:example.com ~all
If example.com does not resolve, the result is fail. 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.
In the following example, the client IP is 1.2.3.4 and the current domain is example.com.
v=spf1 include:example.com ~all
If example.com has no SPF record, the result is PermError.
Suppose example.com’s SPF record were 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”, the include as a whole fails to match; the eventual result is still Fail from the outer directive set in this example
The SPF record for 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 also include the ‘all’ mechanism. If both are present, the ‘redirect’ modifier is ignored. Any ‘redirect’ modifiers beyond the first will be ignored.
For example, this SPF record is published on example.com:
v=spf1 redirect=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.
For example, this SPF record is published on example.com:
v=spf1 mx -all exp=explain._spf.%{d}
and there is a TXT record at explain._spf.example.com with this string:
Emails from example.com must only be sent by its own mail servers.
Over the past decade, it has become increasingly easier to send email. Countless Sources have entered the marketplace, each providing a specialized toolset tailored to address modern day needs of marketers, developers, and small businesses. 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. This limit however 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 email 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 make use of 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 in to the ten maximum lookup condition if you are correctly segmenting different mail streams (eg. transactional, corporate, marketing, etc.) on to discrete name space.
In this sub section “delivery tips’ of the Gmail postmaster site, it is recommended to;
In summary, you should not run in to the 10 lookup maximum. If you do, we’ve outlined some additional strategies and knowledge-base materials on how to navigate.
