Short answer: Start with your domain's SPF, DKIM and DMARC records: since 2024-2025 Gmail, Yahoo and Microsoft send unauthenticated mail to spam or reject it. Then check your mail server's IP against blocklists, verify the IP's reverse DNS (PTR) record, and review your content and complaint rate.
1. SPF, DKIM or DMARC is missing or broken
This is the most common cause. The big mailbox providers now publish explicit sender requirements:
| Provider | Applies to | Key requirements |
|---|---|---|
| Gmail (since February 2024) | All senders to personal Gmail accounts | SPF or DKIM, valid forward and reverse DNS, TLS, spam rate reported in Postmaster Tools below 0.3% |
| Gmail bulk senders | Senders of 5,000+ messages a day to personal Gmail accounts | SPF and DKIM and DMARC (at least p=none), From domain aligned with SPF or DKIM, one-click unsubscribe for marketing mail |
| Yahoo / AOL (since February 2024) | All senders, stricter for bulk | Essentially the same rules as Gmail |
| Microsoft (since May 2025) | Domains sending 5,000+ messages a day to Outlook.com, Hotmail and Live addresses | SPF, DKIM and DMARC (at least p=none, aligned); non-compliant mail goes to Junk or is rejected with 550 5.7.515 |
- Enter your domain in the SPF, DKIM, DMARC checker.
- Your SPF record must list every service that sends mail for you (your mail provider, website, invoicing system, newsletter tool) in a single record.
- DKIM must be enabled at your mail provider: Google Workspace, Microsoft 365, Zoho Mail, cPanel.
- Publish a DMARC record;
v=DMARC1; p=none; rua=mailto:dmarc@yourdomain.comis enough to start.
2. Your mail server's IP is on a blocklist
If your server's IP is listed by Spamhaus, SpamCop, UCEPROTECT or similar, receiving servers will send your mail to spam or reject it with 550 5.7.1. Check with the blacklist check tool. If you are listed, find the source first (usually a compromised account), then follow that list's own process: Spamhaus, SpamCop, UCEPROTECT, Microsoft.
3. Reverse DNS (PTR) is missing or doesn't match
The IP address of the sending server needs a PTR record, and it should match the name the server announces in HELO/EHLO and resolve back to the same IP (forward-confirmed reverse DNS), for example 203.0.113.10 → mail.yourcompany.com → 203.0.113.10. Without it, Gmail often answers with a temporary 421 4.7.0 or files the message as spam. PTR records are set by whoever owns the IP (your hosting, cloud or internet provider), not in your domain's DNS. On shared hosting, it is the host's responsibility.
4. Content and formatting look like spam
- Messages that are a single large image with little text.
- URL shorteners (bit.ly and similar) and links whose visible text differs from the real destination.
- All-caps subject lines, piles of exclamation marks, "FREE" and "URGENT" style wording.
- Risky attachments such as password-protected ZIPs or macro-enabled Office documents.
- Bulk mail without an unsubscribe option. Gmail and Yahoo require one-click unsubscribe (
List-UnsubscribeandList-Unsubscribe-Postheaders, RFC 8058) from bulk senders.
5. Recipients are clicking "Report spam"
Every spam complaint directly lowers your domain and IP reputation. Gmail expects the spam rate shown in Postmaster Tools to stay below 0.3%, and recommends staying under 0.1%. Only mail people who asked for it, never use purchased lists, and remove addresses that bounce with 550 5.1.1. If you send a lot to Gmail, monitor your reputation in Google Postmaster Tools; for Outlook.com, use Microsoft SNDS.
6. New domain or new IP
A freshly registered domain or a new server IP has no reputation yet. Avoid high volumes in the first weeks and ramp up gradually (warming). Sending day-to-day business mail from a different infrastructure, or at least a different subdomain, than your marketing campaigns keeps a campaign problem from hurting your everyday correspondence.
How do I test the result?
- Send a test message to your own Gmail and Outlook.com addresses.
- In Gmail, open the message and choose ⋮ → Show original; you should see
PASSfor SPF, DKIM and DMARC. - Paste the headers into the email header analyzer to read the details.
- If the message went to spam, Gmail usually explains why in a banner above it (for example "This message is similar to messages that were identified as spam in the past").
This guide is about your outgoing mail. Stopping spam and phishing that arrives in your mailboxes is a different job: how to stop spam emails.
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Frequently asked questions
Only Outlook/Hotmail puts my mail in spam; Gmail is fine. Why?
Microsoft runs its own reputation system, which does not map one-to-one to public blocklists. Enroll in SNDS to see how Microsoft rates your IP, and use the Outlook.com sender support form if needed.
All my records are correct but I still land in spam.
Authentication is necessary but not sufficient. IP and domain reputation, content and recipient engagement (opens, replies, spam reports) also matter. Asking regular correspondents to add you to their contacts helps too.
I'm on shared hosting. Could another customer be the problem?
Yes. If another account on the same IP sends spam, the IP can be blocklisted. Ask your host for a different IP, or send business mail through a dedicated email service.