Email Deliverability: Why Emails Go to Spam and How to Fix It

Email deliverability is the share of your email that reaches the inbox instead of the spam folder or a bounce. When emails are going to spam, the cause is almost always one of six things: missing authentication, a weak list, spam complaints, low engagement, content that looks off, or sudden volume. The fix runs in the same order. Authenticate your domain, mail only people who asked, make leaving easy, drop dead addresses and grow volume slowly. Then test with real inboxes and watch your complaint rate.

Why are my emails going to spam?

Mailbox providers judge every message on three questions. Can they confirm who sent it? How do recipients react to this sender? And does the sending pattern look normal? Google puts the second one plainly: if messages from your domain are often reported as spam, future messages are more likely to land there too, and over time those reports lower your domain's reputation ^[1]. Reputation is memory. One careless send can shape the next ten.

Reputation attaches to your domain and to the sending IP. On a shared IP, which most email platforms use, Google notes that every sender's activity affects the reputation of all the others on that address. That is one reason platforms police their customers so closely.

What Gmail and Yahoo require from senders today

Since February 1, 2024, Google requires everyone who sends to Gmail accounts to set up SPF or DKIM, have valid forward and reverse DNS records, use a TLS connection and keep the spam rate in Postmaster Tools below 0.3%. Anyone sending more than 5,000 messages a day to Gmail accounts must also set up SPF and DKIM together, publish DMARC (the policy can be none), align the From: domain with SPF or DKIM, and support one-click unsubscribe plus a visible unsubscribe link in the body. Google's own target is stricter: stay below 0.10% and never reach 0.30%.

Yahoo's sender requirements match closely. All senders need SPF or DKIM and a spam rate below 0.3%. Bulk senders need both SPF and DKIM, a DMARC policy of at least p=none, one-click unsubscribe, a visible unsubscribe link, and they must honor unsubscribes within 2 days ^[2]. Yahoo also calculates the spam rate on mail delivered to the inbox, not on everything you sent. It tells senders not to buy lists or pre-check the opt-in box on a sign-up form.

The law sits under all of this. US commercial email falls under CAN-SPAM. The FTC's CAN-SPAM Rule defines a valid physical postal address as the sender's current street address, or a PO box or private mailbox the sender has accurately registered. The same rule bars senders from requiring a fee, any information beyond the email address and opt-out preferences, or any step beyond a reply email or a visit to a single web page in order to opt out ^[3].

How to run an email deliverability test

A useful email deliverability test uses real inboxes and your real sending setup. A preview button inside your email tool tells you how the email looks, not where it lands.

1. Build a small seed list. Create free personal addresses at Gmail, Yahoo, Outlook.com and iCloud, and add your own work address.
2. Add the seeds to your actual list and send them a real campaign from your normal sending domain.
3. Check the folder in each inbox. Note inbox, a tab such as Promotions, or spam.
4. Read the headers. In Gmail, open the message, click More next to Reply, then Show original, and the full header opens in a new window ^[4]. Look for pass results on SPF, DKIM and DMARC.
5. Open Google Postmaster Tools for your domain. Google says it shows when recipients mark your mail as spam, why messages might not be delivered, and your domain or IP reputation.
6. Repeat after every change to records, sending platform or volume.

Treat a seed test as a smoke alarm, not a verdict. Seeds have no history with you, while real subscribers do, and providers weigh that history. A Promotions tab is still the inbox. Spam placement across several seeds, or a failed authentication line in the header, is the real warning.

How to improve email deliverability, step by step

Work in this order. Each step removes a reason for providers to doubt you, and later steps help little while an earlier one is broken.

1. Authenticate your sending domain

Publish SPF and DKIM for the domain in your From: address, then a DMARC record. Start DMARC at p=none with a reporting address so you can see who sends as you. Google requires DKIM keys of at least 1024 bits for personal Gmail accounts and recommends 2048 bits where your provider allows it. Our SPF, DKIM and DMARC setup guide walks through the records at your domain host.

2. Mail only people who asked

Yahoo recommends a confirmation email that new subscribers click to confirm their opt-in, which keeps out uninterested people, fake addresses and most robots. Tell people at sign-up what you will send and how often. Then keep that promise. Yahoo warns against switching a weekly list to daily mail.

3. Make leaving easy

An unsubscribe is far better for you than a spam report. Use one-click unsubscribe, keep a visible link in the body, and process requests fast. Google adds that people should be able to review their subscriptions and leave one list or all of them at once.

4. Clean the list

Remove addresses that bounce. Google suggests automatically unsubscribing recipients with multiple bounces and considering the same for people who never open or read. Yahoo suggests a periodic reconfirmation email to inactive subscribers. A smaller, engaged list beats a big, sleepy one.

5. Grow volume slowly

Send at a consistent rate and avoid bursts. Google advises starting with low volume to engaged users and increasing slowly; doubling your volume overnight can bring rate limits or a reputation drop. If bounces or deferrals start, cut volume until the errors fall, then climb again.

A worked example

Illustration with round invented inputs: a 2,000-subscriber newsletter, 1,000 readers on Gmail and 1,000 on Yahoo, every copy delivered to the inbox. Postmaster Tools counts only mail sent to Gmail users, so a single spam report already equals Google's 0.10% target, and 3 reports reach the 0.30% line Google says never to reach. Yahoo's math is the same: 3 reports among its 1,000 inbox deliveries equal 0.3%. A handful of annoyed readers is enough.

Common mistakes that send emails to spam

Most deliverability trouble is self-inflicted, and the same few habits cause it again and again.

• Importing an old list and mailing it all at once. Dead addresses bounce, forgotten subscribers complain, and the volume spike looks suspicious.
• Sending bulk mail from a personal mailbox. Mixing marketing with everyday mail puts your personal reputation at risk; Yahoo advises keeping bulk mail apart from user and transactional mail.
• Hiding or slowing the unsubscribe. People who cannot leave easily hit report spam instead.
• Changing the format and volume together. Google suggests ramping up a new format gradually as well.
• Testing in one inbox. Your own inbox knows you and proves little.

When deliverability gets hard, and tools that help

It gets hard when the list is old, when several tools send as your domain, or when complaints keep rising after the basics are fixed. It also gets hard when your platform steps in. Email platforms watch their shared reputation and can pause an account. Brevo, for example, publishes suspension thresholds of over 2% hard bounces, 1% unsubscribes or 0.2% complaints on a campaign. Treat numbers like these as that vendor's rules, not industry law.

No platform can promise the inbox. Deliverability is mostly sender practice, and the right tool makes the good habits automatic. Kit recommends authenticating a verified sending domain and adding a DMARC record, and documents both. Mailchimp turns hard-bounced addresses into Cleaned contacts that get no further email.

For a side-by-side look at list verification, warm-up and monitoring options, see our guide to email deliverability tools.

The bottom line

Email deliverability is earned, not configured once. Authentication gets you through the door: SPF, DKIM and DMARC for every sending domain, as Google and Yahoo both recommend. After that, the inbox follows behavior. Mail people who asked, let them leave in one click, remove bounces and dormant addresses, and grow volume gradually. Test with seed inboxes after every change and watch your spam rate in Postmaster Tools. Keep it under 0.10%, and treat 0.3% as the wall Gmail and Yahoo both draw.