Authentication is no longer advice. SPF, DKIM and DMARC are checked, and mail that fails them is treated as suspect before a human sees it.
Complaint and bounce rates are watched at the domain level, and the response is no longer a delay. Gmail moved from deferring questionable mail to refusing it outright, and Microsoft tightened in the same direction. A domain that crosses the line does not get a warning.
Which is why volume per mailbox matters more than volume in total. One domain sending a lot is a pattern. Many domains each sending a little is a business.
What do Gmail's sender rules require?
Every sender needs SPF or DKIM, valid forward and reverse DNS, a TLS connection, and a spam rate in Postmaster Tools below 0.3%, with 0.10% as the level Google asks senders to stay under. Senders of 5,000 or more messages a day to Gmail accounts need SPF, DKIM and DMARC together, a From domain aligned with SPF or DKIM, and one-click unsubscribe on marketing and subscribed messages.
| Requirement | All senders | 5,000 or more a day |
|---|---|---|
| SPF, DKIM | SPF or DKIM | both |
| DMARC, From aligned with SPF or DKIM | not listed | required |
| Forward and reverse DNS, TLS | required | required |
| Spam rate in Postmaster Tools | below 0.3%, target under 0.10% | below 0.3%, target under 0.10% |
| One-click unsubscribe | not listed | marketing and subscribed messages |
The 5,000 count covers the sending domain together with its subdomains, and once a domain is treated as a bulk sender that status stays.
Why build to the bulk rules below the threshold?
At five meetings a week the whole pack sends 3,000 emails a week, so the bulk rules do not bind it. They are still the floor: SPF, DKIM and DMARC are verified on every domain before anything sends.
Sources: Google, Email sender guidelines; Google, Email sender guidelines FAQ.
One domain carries one mailbox. Fifty a day per mailbox, never more, with warm-up counted inside that fifty. Sending stops on its own if bounces pass 2% or complaints pass 0.1%.