Why do newsletters land in spam? Causes and fixes

You spent hours on your newsletter, and then a subscriber tells you they found it in their spam folder. Or worse: the confirmation e-mail of your app never reaches new customers at all. Spam filters are not a black box. They follow rules you can understand, and most causes can be fixed with a few settings.

How spam filters decide

Large mailbox providers such as Gmail, Outlook or Yahoo weigh many signals for every message. Roughly, they fall into four groups:

• Authentication: may this server send for the sender's domain, and was the message changed on the way?
• Reputation: how have messages from this domain and this IP address behaved in the past?
• Content and structure: does the message look like typical spam?
• Recipient reactions: are messages opened, answered, deleted or marked as spam?

You control the first three groups directly. The fourth follows once the rest is right and your newsletter is genuinely wanted.

Reason 1: missing authentication

Without SPF, DKIM and DMARC a receiver cannot check whether a message really comes from your domain. Since February 2024, Gmail and Yahoo require all three from bulk senders who send several thousand messages a day. Small senders benefit clearly as well.

Our guide SPF, DKIM and DMARC explained covers what each record does. The short version:

• SPF lists the servers allowed to send for your domain.
• DKIM signs every message cryptographically.
• DMARC decides what happens to messages that fail both.

Reason 2: the domains do not match

A common stumbling block: you send through a newsletter service that signs your mail, but with its own domain. The sender says [email protected], while the DKIM signature belongs to mailing-service.example. For DMARC that is not a pass, because at least SPF or DKIM has to match the visible sender domain. Experts call this "alignment".

Almost every service offers the fix: you add the service's DKIM keys for your own domain to your DNS, usually as CNAME records. From then on the service signs in the name of example.com.

Reason 3: no unsubscribe link

Recipients mark a newsletter without an easy way out as spam, because that is the fastest way to get rid of it. Every one of those reports hurts your reputation.

Besides the link in the text, Gmail and Yahoo expect bulk senders to include the List-Unsubscribe header with one-click unsubscribe (List-Unsubscribe-Post: List-Unsubscribe=One-Click). That puts a dedicated "Unsubscribe" button in the inbox. Good newsletter services set these headers automatically; with a home-grown system you have to remember them.

Reason 4: a poor address list

Bought lists, old addresses and missing confirmation are poison for deliverability. Some providers turn abandoned mailboxes into spam traps, and anyone who writes to many undeliverable addresses looks like a spammer.

Here is how to keep your list clean:

• Double opt-in: new subscribers confirm their address with a link. It also gives you proof of consent.
• Handle bounces: remove addresses that are permanently undeliverable right away.
• Prune inactive subscribers: anyone who has not opened anything for many months gets one last check-in and is removed afterwards.

Gmail recommends keeping the spam complaint rate below 0.1 percent; at 0.3 percent it becomes critical.

Reason 5: the content

Content filters matter less today than authentication, but they still count. Typical triggers:

• A single large image and hardly any text
• No plain text version, only HTML
• Many links, especially through third-party link shorteners
• Very large messages: Gmail clips messages above roughly 102 KB of HTML and hides the rest, often including the unsubscribe link.
• Missing standard headers such as Message-ID or Date, which point to a poorly built sending system
• An empty or overlong subject

Reason 6: a new domain without history

A freshly registered domain or a new IP address has no reputation yet. Sending to ten thousand recipients from it right away looks suspicious. Start with small batches to your most active subscribers and increase the volume over a few weeks. This "warm-up" is a must for new senders.

Checklist before your next send

Check	Goal
SPF	exactly one record that includes your sending service
DKIM	signature with your own domain
DMARC	record present, later quarantine or reject
Unsubscribe	link in the text and List-Unsubscribe header
List	double opt-in, bounces removed
Content	text and HTML version, below 100 KB

Test before you send

Instead of guessing, send your newsletter to a test address first. Our mail tester gives you an address that stays open for one hour. As soon as your message arrives, it checks SPF, DKIM, DMARC and their alignment, the DNS records of your domain, the unsubscribe header, the share of text and images, size and subject. You get a score and, for every problem, a concrete hint on what to do.

Test your newsletter with the mail tester now – free and without signing up.