What is a source, and how do you know which service it is?

On every plan

A source is one service that sends mail as your domain, such as Mailchimp or Microsoft 365. When we cannot name the service, a source is a group of sending addresses that belong together, or one address on its own. Each row on a domain's Sources page is one source.

How we name a source

A report from a mailbox provider names the address each message came from, not the service behind it. We work out the service from the address. The first of these that matches wins:

The address is inside a range the service authorizes in its own SPF record, or a range we list for it.
Mail from the address carries a passing DKIM signature by a domain that belongs to the service, such as mcsv.net for Mailchimp.
The address's reverse DNS name matches a pattern the service uses.
The address is in the service's network, by its autonomous system number (ASN).

The top of a source's page says which of these matched, and on what: the SPF range, the DKIM domain, the reverse DNS name or the network.

What "probably" means

An SPF range or a DKIM signature is the service's own statement: it authorized the address, or it signed the mail. A reverse DNS name or a network is weaker evidence, because other companies can send from the same network. A source named by its reverse DNS name or its network has the word "probably" after its name, and its page gives the reason.

Sources we cannot name

When nothing matches, we still group the addresses, so that you do not read a list of numbers:

by the organizational domain of their reverse DNS name: mail1.hosting.example.net and smtp.example.net both go in the group example.net;
by the network operator, when an address has no reverse DNS name;
as a single address, when it has neither.

The line under the source's name says which kind of group it is.

The services we know

We know 67 services by name. For each one we keep the SPF record it asks you to include, the domains it signs DKIM with, its reverse DNS patterns and its networks.

Once a week we read each service's SPF record again, so its ranges follow the service's own changes. After that, every address still in an unnamed group gets another look, and an address that now matches a service moves to it.

Name a source yourself

An admin can give any source a name of its own on the source's page, such as "Office printer". The name shows in place of the one we worked out, and an admin can remove it again. A source you have named counts as identified when we check readiness.

Unnamed sources that always fail

A source we cannot name whose mail never passes DMARC is probably someone else sending as you. The readiness rule leaves such sources out, because a stricter DMARC policy is what stops them.

If you recognise one, it is a service of yours that is not set up for your domain. Set it up so that SPF or DKIM aligns with your domain, then name the source. How we explain each source shows the sentence and the action each case gets.

Published 27 October 2026 · Checked 27 October 2026