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Email deliverability: keep your email out of spam

For anyone who sends to a list: how to set up your domain, how much to send, and how to fix bounces and spam-folder problems.

Since 2024, Gmail and Yahoo have required bulk senders to authenticate their email with SPF, DKIM and DMARC, keep spam complaints low and offer one-click unsubscribe. Microsoft added similar authentication rules for Outlook.com in May 2025. Getting these basics right matters more than any trick. Start with the deliverability checklist, then use the free tools to check your domain.

Set up your domain

• For senders

  Email deliverability checklist

  Everything Gmail, Yahoo and Outlook check before they decide inbox or spam, in one list.

• For senders

  Primary domain vs cold email domain

  Why outreach usually goes out from a separate domain, how to name it, and how to set it up properly.

• For senders

  What is BIMI?

  How brands get their logo next to their emails in Gmail, Yahoo and Apple Mail, what it requires, and whether it is worth it.

Sending

• For senders

  How many cold emails per day?

  Safe daily limits per inbox and per domain, Gmail and Workspace hard limits, and how to ramp up.

• For senders

  Email many people without showing addresses

  Use Bcc in Gmail and Outlook, the recipient limits, and when to switch to mail merge or a newsletter tool.

• For senders

  Subject lines and spam words

  What really sends email to spam, the words and habits to avoid, and subject lines that get opened, with before-and-after examples.

• Email help

  Why attachments go to spam

  Why emails with attachments are treated as risky, the file types Gmail and Outlook block, and how to send files without landing in spam.

Bounces and list quality

• For senders

  Email bounce-back messages

  What "Address not found" and "Delivery has failed" mean, how to read them, and how to fix the cause.

• For senders

  Soft bounce vs hard bounce

  The difference, the error codes behind each, and exactly what to do with each type.

• For senders

  How to clean an email list

  A 7-step routine to remove bad, risky and inactive addresses before your next send.

• For senders

  Email list hygiene best practices

  The habits that keep a list healthy all year: double opt-in, sunset policies, and avoiding spam traps.

Background

• For senders

  The history of email marketing

  From the first spam in 1978 to Mailchimp, Klaviyo and today's sender rules: every major email marketing service and the billion-dollar deals.

Technical reference

• For senders

  SMTP conversation explained

  A real SMTP session line by line: EHLO, STARTTLS, AUTH, MAIL FROM, RCPT TO, DATA and the reply codes in between.

• For senders

  Email header fields explained

  What every common email header means, from Received and Return-Path to Message-ID, References and List-Unsubscribe.

• For senders

  TLS vs STARTTLS

  Port 465 vs 587, implicit TLS vs STARTTLS, and why encryption in transit is not the same as end-to-end encryption.

• For senders

  Why forwarding breaks SPF and DMARC

  Why forwarded mail fails SPF, when DKIM survives, what ARC and SRS do, and how to forward without losing mail.

• For senders

  Email encoding explained

  MIME, Base64, quoted-printable and UTF-8: how email carries attachments and special characters, and why garbled text happens.

Questions people ask

Why do my emails go to spam?

The usual causes are missing SPF, DKIM or DMARC, sending too much too fast, a list with many bad addresses, or content that looks like spam. The checklist covers each one.

Do I need DMARC?

If you send in bulk to Gmail or Yahoo, yes: at least p=none. Use the DMARC generator.