How to configure SPF in Postfix

SPF, or Sender Policy Framework, is a way for domain owners to say: “these are the email servers my domain sends mail from; anyone else is attempting to impersonate me.” These days, a lot of email services implement it in some way (both for receiving and sending email), and, if you administer your own server, it’s quite easy to implement, as we’ll see.

There are actually two parts to SPF: configuring it for your domain (so that your outgoing emails have a better chance of being accepted and emails impersonating your domain have a better chance of being refused), and configuring your email server to check whether incoming emails come from an SPF authorized server or not. Without further ado:

1- Configuring SPF for your domain’s outgoing mail:

This is actually done in DNS, not in the email server. Simply add a TXT record for your domain, with something like this:

v=spf1 a:mailserver1.domain.com a:mailserver2.domain.com -all

There are other configuration parameters, such as getting the servers from the domain’s MX record (add “mx“), and copying another domain’s authorized servers (“include:otherdomain.com“). And, of course, if you only have one server sending mail, you don’t need two “a:” entries.

Pay special attention to the “-all” part at the end: that instructs other servers to refuse any email apparently coming from your domain that isn’t authorized for it. That should ideally be the final configuration, but if you’re doing this for the first time you may want to change it to “~all“, which means “softfail”: it suggests to the other side that the mail is probably not legit, but that it shouldn’t refuse it outright because of it. After testing it for a while, if you see no problems then you can change it to “-all“.

2- Configuring SPF checking for incoming mail in Postfix:

I’ll assume you already have a working Postfix server (hopefully already using TLS encryption, hint hint), and that you want to respect any “-all” configurations (see above) in incoming mail — in other words, if a domain says “all my mail comes from server X, refuse anything else” and you receive an email purportedly from that domain, but from a server Y, it’s refused (for “~all“, it’d generate a warning).

First, install pypolicyd-spf. On a Debian-based system, the package (as of this writing) still has its old name, so enter the following:

apt install postfix-policyd-spf-python

Edit /etc/postfix/master.cf, and add:

# spf
policyd-spf  unix  -       n       n       -       0       spawn
        user=policyd-spf argv=/usr/bin/policyd-spf

Then edit /etc/postfix/main.cf, and look for the “smtpd_recipient_restrictions” section. That section will probably begin by accepting mail from your networks and from authenticated users, then rejecting non-authorized relaying, possibly followed by some white- or blacklists, and maybe a couple of filters, finally ending with “permit“. Before that “permit“, add:

    check_policy_service unix:private/policyd-spf,

(note the comma at the end; without it, Postfix wouldn’t read the rest of that section.)

IMPORTANT: while you’re still in testing, it may be a good idea to add “warn_if_reject” to the beginning of the previous configuration line. With it, Postfix will still log SPF failures, but won’t actually reject an email because of it. When you’re certain everything is fine, you can then remove that prefix (and then reload Postfix, of course).

Add also the following to that configuration file, outside of any section:

policy-spf_time_limit = 3600

And you’re done. Reload or restart Postfix, and check your logs to see if everything is working as intended.

Note that other tools, such as SpamAssassin, may also use SPF to weigh whether an incoming mail is more or less likely to be spam. That, however, is unrelated to the Postfix configuration above, which does a binary “block/don’t block”.

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