The rules side by side
| Requirement | Gmail: every sender | Gmail: 5,000+ a day | Outlook.com: 5,000+ a day |
|---|---|---|---|
| SPF | SPF or DKIM | Required | Required |
| DKIM | SPF or DKIM | Required | Required |
| DMARC | Recommended | Required | Required, at least p=none, aligned |
| Reverse DNS (PTR) for the sending server | Required | Required | — |
| TLS connection | Required | Required | — |
| Spam rate | Below 0.10%, never 0.30% | Below 0.10%, never 0.30% | — |
| Unsubscribe (marketing mail) | Easy to unsubscribe | One-click, required | Working link, recommended |
| What happens if not met | Spam folder or refusal (5.7.26) | Spam folder or refusal | Refused: 550 5.7.515 |
Sources: Google’s email sender guidelines and Microsoft’s announcement for high-volume senders (enforced from 5 May 2025), read on 30 October 2026.
For a normal business mailbox
A business that sends ordinary mail, not newsletters, stays far below 5,000 a day. The rules still matter: a domain without SPF, DKIM and DMARC is easier to fake and its mail is trusted less. The good news is that all three are free, and your provider does the hard part.
- Publish the SPF, DKIM and DMARC records your provider gives you (see SPF, DKIM and DMARC explained).
- Send newsletters and bulk mail through a service built for it, never from your own mailbox.
- Never buy lists or mail people who did not ask to hear from you.
Questions people ask
What does Gmail require from every sender?
SPF or DKIM for your domain, a sending server with matching reverse DNS (PTR), a TLS connection, and a low spam rate: Google asks senders to stay below 0.10% and never reach 0.30% in Postmaster Tools.
What changes at 5,000 emails a day?
Both Gmail and Outlook.com then require SPF, DKIM and DMARC (at least p=none, aligned with SPF or DKIM). Gmail also requires one-click unsubscribe on marketing and subscribed mail.
What is error 550 5.7.515?
Outlook.com’s refusal for a high-volume sender whose domain does not pass the required SPF, DKIM and DMARC checks. Fix the records; the message itself is fine.
What is error 5.7.26 from Gmail?
Gmail refused a message that was not authenticated: SPF or DKIM failed (or DMARC failed for your domain). Check your records with a domain checker and that your provider signs with DKIM.
I send fewer than 5,000 emails a day. Can I skip DMARC?
You can, but it is a bad idea. DMARC is what stops strangers sending mail as your domain, and receivers trust authenticated mail more. Starting with p=none costs nothing.
Try it on your own domain
Set up a real mailbox at your own domain, with every record checked for you. No card needed.