Why are my emails going to spam?

Most advice on this question is about wording — avoid the word "free", cut the exclamation marks, do not use red text. That advice is largely folklore, and it survives because it is easy to act on. The real answer is duller and more fixable, and both of the companies that decide the outcome have published it.

The one number they publish

Google's sender guidelines and Yahoo's sender best practices set exactly one numeric threshold between them:

Keep your spam complaint rate below 0.3%. Google adds that you should aim below 0.10% and never reach 0.30%.

That is three complaints per thousand messages to cross the line, and one per thousand to be comfortable. It is a smaller margin than most senders realise, and it is the number to watch above all others — Gmail reports yours in Postmaster Tools.

1. Authentication, which is now mandatory

This is where most spam-folder problems actually live, and it is the part that is no longer optional.

• SPF lists which servers may send mail using your domain.
• DKIM signs each message cryptographically so the receiver can confirm it was not altered and really came from you. Yahoo asks for a key of at least 1024 bits.
• DMARC tells receivers what to do when SPF and DKIM disagree, and where to send reports. A policy of p=none satisfies both Google and Yahoo — you do not need to start by rejecting mail.

Google requires SPF or DKIM from every sender, and all three together from anyone sending more than 5,000 messages a day to Gmail addresses. Yahoo requires SPF and DKIM plus a valid DMARC policy from bulk senders. Yahoo additionally expects valid forward and reverse DNS for your sending IPs.

If you send through a platform such as Mailchimp, Klaviyo or Brevo, this is configured once in your DNS and then stays done. If you are not certain yours is correct, that is the first thing to check, because nothing else you fix will compensate for it.

2. Unsubscribing, which is also mandatory

Both providers require marketing mail to support one-click unsubscribe, with a clearly visible unsubscribe link in the message body. Yahoo names the RFC 8058 method specifically and requires unsubscribes to be honoured within two days.

There is a self-interested reason to make this easy rather than to bury it. A reader who cannot find the unsubscribe link presses the spam button instead, and that lands directly on the 0.3% figure above. A hidden unsubscribe link does not retain a subscriber — it converts a departure into a complaint.

3. Reputation, which is earned and lost slowly

Mailbox providers keep a running judgement of your sending domain. Three things move it:

• Spam complaints. The heaviest signal, and the one with a published limit.
• Sending to addresses that do not exist. A pattern of hard bounces is measurable evidence that you do not know who is on your list, and providers read it that way. This is the connection between list quality and spam placement, and it is why bounce rate matters more than the small fraction that bounced.
• Spam traps. Abandoned addresses that providers reactivate to catch senders who never clean their lists. These often do not bounce at all — they accept quietly and damage you invisibly, which is what makes them worse than a bounce.

Reputation is attached to your domain, not to the individual message, which is why one bad send can affect the deliverability of everything that follows it — including mail to people who very much want to hear from you.

4. Engagement, which you cannot fake

Providers watch whether recipients open, reply, and move mail out of spam, and they watch whether they delete without reading. A list of people who genuinely want your mail is the strongest deliverability asset there is.

The practical implication is uncomfortable: a smaller engaged list outperforms a larger indifferent one, not slightly but substantially. Mailing people who stopped caring two years ago does not merely fail to reach them, it degrades your ability to reach everyone else.

Yahoo's own advice is to use confirmed opt-in and to periodically send a reconfirmation to inactive subscribers. That is the provider telling you directly that a smaller, willing list is what it wants to see.

What about the wording?

Largely a myth, and a durable one. Modern filters weigh authentication, reputation and engagement far above vocabulary. A properly authenticated sender with an engaged audience can write "free" in a subject line without consequence. A sender with broken SPF and a 0.5% complaint rate will land in spam having written nothing but courtesy.

The content-related things that do still matter are structural rather than lexical:

• A large image with almost no text, which is a spammer pattern because it defeats text analysis.
• Link shorteners, which hide the destination and are used heavily in abuse.
• A From address at a free consumer domain rather than your own.
• Sudden large changes in sending volume, which look like a compromised account.

Fix it in this order

• Check SPF, DKIM and DMARC. Free to verify, and the most common single cause.
• Make the unsubscribe link obvious and honour it within two days.
• Find your complaint rate in Google Postmaster Tools and get it under 0.10%.
• Remove people who have not engaged in a year, after one honest attempt to re-engage them.
• Check the list before you send, so hard bounces never accumulate against your domain in the first place.
• Keep volume steady. A tenfold jump after months of quiet reads as a compromise.

Notice where list checking sits. It is a real factor and it is genuinely worth doing, but it comes after authentication, because a list of perfectly valid addresses sent from an unauthenticated domain still lands in spam.

Where this leaves you

If your mail is going to spam, the odds strongly favour authentication or complaint rate over anything you wrote. Both are checkable today, and both have published requirements you can read yourself rather than take on trust.

For the list-quality part, ListCheckup checks a list before you send, shows the price before anything is spent, never bills for duplicates or malformed rows, and gives every result a plain-English reason — including an honest undetermined where nobody can know. It opens in the coming weeks. Leave your address and we will tell you the day it does.

Related: what counts as a good bounce rate, and hard bounce vs soft bounce.