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This page is for mail that is delivered but lands in the spam folder.
If your mail is being rejected rather than filtered — bounces with 550, or nothing arrives at all — that is a different problem. See Outbound email blocked: relay locks and IP reputation.

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Start with authentication

This is the one that matters most. Gmail and Yahoo both require authenticated mail, and unauthenticated mail from a custom domain is filtered or refused as a matter of policy rather than judgement. Set up SPF, DKIM and DMARC before changing anything else — see Email authentication. Most “my mail goes to spam” cases end here. Also worth having: reverse DNS for your sending domain. See Configuring reverse DNS.

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Then the message itself

Filters score content as well as sender reputation:
  • Send plain text, or multipart MIME with a text/plain part, rather than HTML only
  • If you do send HTML, make sure it is valid markup — the W3C validator will tell you
  • Avoid strings of exclamation marks and currency symbols in subject lines
  • Avoid “test” and placeholder text. Test messages are among the most likely things to be filtered, which makes them a poor way to check whether filtering is happening

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Reputation takes time to build

Sender reputation is built from how recipients behave. Messages that get opened and read, and messages dragged out of the spam folder into an inbox, are strong positive signals to the receiving provider. That works in reverse too: sending to stale lists, or to addresses that bounce, damages it. Changes take several days to have an effect. Fixing authentication today will not move your mail out of spam this afternoon.

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Provider postmaster programmes

Microsoft runs two schemes worth joining if you send to Outlook.com, Hotmail or Live addresses in any volume:
  • Junk Email Reporting Partner (JMRP) — tells you when recipients mark your mail as junk
  • Smart Network Data Services (SNDS) — shows you the reputation data Microsoft holds on your sending IP
Both are at postmaster.live.com. Keep the verification number you are given; you will need it for any later correspondence with Microsoft. Google publishes its sender guidelines and expects senders to follow them.

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Sending in volume

If you are sending newsletters, campaigns or a high volume of transactional mail, a shared hosting relay is the wrong tool and no amount of content tuning will fix it. Use a dedicated service — Mailchimp, SendGrid, Mailgun or Amazon SES — which handles reputation, list hygiene and unsubscribes for you. See Using external SMTP servers to send email and Email sending limits.

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Blacklists

If your mail is being rejected because an IP address appears on a blocklist such as Spamhaus or SpamCop, that is covered on Outbound email blocked, which has the delisting routes. Two things worth knowing:
  • If you send from a dynamic IP address — from your own connection rather than our servers — a previous holder of that address may have got it listed. Ask your ISP for a new address, or send through our servers, or through your ISP’s SMTP server
  • We are not affiliated with any blocklist organisation and have no control over their listings or policies