The large mailbox providers publish requirements for senders above a volume threshold. They converged on broadly the same list, which makes it a single checklist rather than a per-provider exercise.
Who counts
The commonly stated threshold is 5,000 messages a day to a single provider’s users, measured per sending domain. Two things about that number catch people:
- It is per provider, not in total. A domain sending 5,000 a day to Gmail users qualifies regardless of its overall volume.
- Crossing it once can be enough. A single large announcement can put a domain in scope, and the requirements then apply to everything it sends.
The requirements
| Requirement | Detail |
|---|---|
| SPF and DKIM | Both, aligned with the From domain. Not one or the other. |
| A DMARC record | p=none satisfies the letter of it. Enforcement is the point of having one. |
| Forward-confirmed reverse DNS | On every sending host. |
| TLS for transmission | STARTTLS on outbound connections. |
| One-click unsubscribe | The List-Unsubscribe-Post header, not merely a link in the body. |
| Unsubscribes honoured within two days | Measured from the request, not from the next list build. |
| Complaint rate below 0.30% | With 0.10% given as the level to stay under in practice. |
| No impersonation of the provider | Do not send as gmail.com from your own infrastructure. |
One-click unsubscribe
The part most often implemented incorrectly. It needs two headers, and the second is the one that makes it one-click:
List-Unsubscribe: <https://example.com/u/abc123>, <mailto:u+abc123@example.com> List-Unsubscribe-Post: List-Unsubscribe=One-Click
- The client POSTs to the URL. It must unsubscribe on the POST alone.
- No confirmation page, no login, no preference centre in the path.
- A URL that returns a page asking the recipient to confirm does not satisfy it.
- The
mailto:alternative is optional and still worth including.
These are the floor, not a programme
Everything on that list is achievable in a week by a competent team, and meeting it earns you nothing beyond not being penalised. The complaint-rate line is the only item that is a continuing obligation rather than a one-off configuration, and it is the one that actually determines whether your mail arrives.