Emails go to spam for one of three reasons: they fail authentication (SPF, DKIM, or DMARC), the sending domain or IP has a poor reputation (too many bounces, complaints, or a blocklist entry), or recipients didn't ask for them. Start by reading the authentication results on a message that landed in spam. That one check rules out or confirms the most common cause in a minute.
First, read the authentication results
Send a message to a Gmail address you control, open it, and choose Show original from the message menu. Gmail lists SPF, DKIM, and DMARC with PASS or FAIL at the top, and the full headers below. Look for the Authentication-Results header and note:
- Which domain SPF passed for (
smtp.mailfrom=) - Which domain DKIM signed as (
header.d=) - Whether DMARC passed, and with which policy
If anything says FAIL, start with cause 1. If everything passes, the problem is reputation or content: go to cause 4 onwards.
Cause 1: SPF, DKIM, or DMARC is missing or broken
Gmail requires SPF or DKIM from every sender, and all three plus DMARC from anyone sending 5,000 or more messages a day (Google sender guidelines). Yahoo and Outlook.com have the same bulk rules.
Fix: publish all three for the domain you send from. The DNS records guide lists every value for Amazon SES, and the free deliverability checker tells you which record is missing.
Cause 2: DMARC fails even though SPF passes
SPF checks the envelope sender (the bounce address), not the From address. If your provider uses its own domain for bounces, SPF passes for their domain, which doesn't count for DMARC.
Fix: make sure DKIM signs as your domain (header.d=yourdomain.com), and set up a custom MAIL FROM subdomain so SPF aligns too. SPF, DKIM, and DMARC explained walks through alignment with an example.
Cause 3: Bulk mail without one-click unsubscribe
For marketing and subscribed mail, Gmail and Yahoo require the one-click unsubscribe headers from RFC 8058 plus a visible unsubscribe link, and Yahoo asks that you honor requests within 2 days (Yahoo sender best practices).
Fix: add List-Unsubscribe and List-Unsubscribe-Post: List-Unsubscribe=One-Click to every bulk message. Mailmark adds both to campaign and sequence mail automatically.
Cause 4: Too many spam complaints
Gmail and Yahoo require bulk senders to keep the spam complaint rate under 0.3%, and Google recommends under 0.1%. Complaints come from people who don't remember signing up, got more mail than they expected, or got a different kind of mail than they signed up for.
Fix: check your Gmail spam rate in Google Postmaster Tools. Send only what your signup form promised, make unsubscribing easier than marking as spam, and stop mailing people who haven't engaged in months.
Cause 5: Too many bounces
Sending to addresses that don't exist tells providers your list is old or bought. Amazon SES recommends a bounce rate under 2%, reviews accounts at 5%, and may pause sending at 10% (SES sending review FAQ).
Fix: remove hard bounces immediately, never buy lists, and validate addresses before importing them. The free email list validator checks syntax, disposable domains, and MX records. Mailmark also blocks sends to addresses that previously hard bounced.
Cause 6: A new domain or a sudden volume spike
A domain with no history that suddenly sends thousands of messages looks like a compromised account or a spammer. So does an established domain that jumps far above its normal daily volume.
Fix: ramp up gradually and send to your most engaged recipients first. Mailmark's inbox warming builds a new mailbox's reputation automatically before you send real volume.
Cause 7: Your domain or IP is on a blocklist
Blocklists such as Spamhaus track sources of spam, and many receivers consult them. A listing can send your mail to spam or get it rejected.
Fix: look up your domain and sending IP (the deliverability checker queries several lists at once, and Spamhaus has its own lookup). Fix the cause first, usually a bad list or a compromised account, then follow the list's removal process.
Cause 8: Recipients didn't ask for the mail
This is the cause filters are built to catch, and no technical fix outweighs it. Mail sent to people who never signed up generates complaints, spam-trap hits, and low engagement, and all three feed your reputation.
Fix: send only to people who opted in, ideally with double opt-in, and only the kind of mail they opted into.
A recovery plan, in order
- Authentication: SPF, DKIM, and DMARC all pass, aligned with your From domain.
- Unsubscribe: one-click headers and a visible link on all bulk mail.
- List: hard bounces removed, unengaged recipients paused, nothing bought.
- Blocklists: checked, and delisting requested after fixing the cause.
- Volume: send only to your most engaged recipients for a while, then grow gradually.
- Watch: Postmaster Tools spam rate and your bounce rate after every send.
Recovery is not instant. Providers re-evaluate your domain as they see better mail from it, so keep volume steady and engaged while they do. For the full picture of what keeps mail in the inbox, see the email deliverability guide.
Frequently asked questions
Why do my emails go to spam in Gmail but not Outlook?
Each provider keeps its own reputation data for your domain and uses its own filters. Check Google Postmaster Tools for your Gmail spam rate and domain reputation, and compare the authentication results in each provider's message headers.
Will changing my subject line stop emails going to spam?
Rarely on its own. Authentication, complaint rates, and whether recipients wanted the mail outweigh individual words. Fix those first, then look at content.
How do I check if my domain is on a blocklist?
Look the domain and sending IP up on the blocklist operators' own sites, such as Spamhaus, or use a checker that queries several lists at once, like Mailmark's free deliverability checker.
How long does it take to get out of the spam folder?
There is no fixed time. Providers re-evaluate as they see new mail from you, so fix the cause, then send only to engaged recipients and grow volume gradually.