SPF

SPF, Sender Policy Framework, is a TXT record on your domain that lists which mail servers may send for it. A receiving server looks up the record, compares it against the address in the message envelope, and gets back a pass, a fail, or a softer verdict.

A record looks like this:

v=spf1 include:amazonses.com ~all

include: delegates to another sender's own record — here, the infrastructure Sendly sends through. ~all means anything not listed is suspicious; -all means anything not listed is forged.

Two things people get wrong

SPF checks the envelope sender, not the From address a human sees. Those are different addresses — see Return-Path. This is exactly why SPF alone was never enough, and why DMARC exists.

A domain may have only one SPF record. Two TXT records both beginning v=spf1 is a permanent error rather than a merge, and it fails every check at once. Adding a sender means adding an include: to the record you already have.

There is also a limit of ten DNS lookups while evaluating a record. Each include: counts toward it, and exceeding it is a failure rather than a truncation.

In Sendly

The SPF value you need is shown when you add a domain, and guided DNS setup can publish it and watch it. See Verifying domains.