Why Gmail blocks your email: a short lesson on sender rules

Why does Gmail block an email? A short lesson on the Gmail and Yahoo sender rules.

Think of a building with a gatekeeper. Before someone enters, the gatekeeper checks who they are. Gmail and Yahoo do the same with every email.

Let's follow Lia. She wants to send a sale email from loja.com to 10,000 people on Gmail.

1. Is she a bulk sender? A bulk sender sends email in large volume. On Gmail, this means about 5,000 emails a day to personal accounts. Gmail counts the domain and its subdomains together: 3,000 from mercado.com + 2,500 from promo.mercado.com. If you reach it once, the status does not expire.

All senders need SPF or DKIM, TLS, PTR, and a spam rate below 0.3%. Bulk senders have extra rules.

1. Rule 1: show your ID
   SPF is the list of servers that can send for a domain. DKIM is the email signature. DMARC is the rule that checks both. Bulk senders need SPF, DKIM, and DMARC. The minimum policy is p=none, which only observes.

2. Rule 2: the names must match
   This is alignment. The From domain, which people see, must match the SPF or DKIM domain. Only one of them must match.
   From: loja.com + dkim=pass (envios.com) = it passed, but it is not aligned.
   If SPF is for loja.com, alignment is correct.

3. Rule 3: use a safe entrance
   TLS encrypts the connection, like a sealed bag. PTR is reverse DNS: it links the sending IP to a name. Both rules are for all senders.

4. Rule 4: don't annoy the residents
   Gmail asks for less than 0.1% spam, and it must never reach 0.3%. In 10,000 emails, 10 complaints are 0.1% and 30 are 0.3%. At 0.2% you are still under the limit, but it is time to review your list.

5. Rule 5: offer an easy exit
   Bulk marketing email needs one-click unsubscribe, with the List-Unsubscribe and List-Unsubscribe-Post headers, plus a visible link. A link only in the email body is not enough. Transactional email, like a password reset, does not need one-click.

6. What does the error say?
   Gmail can reply with 421 4.7.x, a temporary limit. Examples:
   421 4.7.30: DKIM did not pass.
   421 4.7.29: TLS is missing.
   421 4.7.32: From is not aligned.
   Retrying without a fix will not help.

7. Are Gmail and Yahoo the same?
   The logic is the same, but some details change. Gmail requires an HTTPS URL for unsubscribe and asks you to honor requests in 48 hours. Yahoo accepts mailto, recommends POST, asks for 2 days, and does not publish a number for bulk senders.

Now be the gatekeeper and test what you learned:
https://thallesaguiar.github.io/email-gatekeeper/en/

email #backend #smtp #programming