Email deliverability guardrails

Use this before any bulk send, and whenever mail starts landing in spam.

The list is the biggest lever

Most deliverability problems are list problems wearing a technical costume.

• Send only to people who asked. A purchased or scraped list damages the domain permanently, and no configuration fixes it.
• Judge engagement on clicks, replies, purchases and product activity, never on opens alone — see below for why opens do not mean what they used to. A contact with none of those for about six months is a risk, not an audience.
• Never suppress on silence alone. Send one re-permission email first and suppress only the people who do not answer it. A quiet subscriber who still buys is not a dead address.
• Remove hard bounces immediately and permanently. Repeated sends to dead addresses are the clearest spam signal there is.
• Watch complaints, not opens. Open rates have been unreliable since mail clients started pre-fetching images.

The setup, checked once

Confirm these exist before the first campaign and never again unless something breaks: SPF, DKIM, and a DMARC record that is at least at p=none and monitored. Send marketing mail from a subdomain, so a bad campaign cannot take the company's transactional mail down with it. A new domain sends to its most engaged people first, in small volumes, for a few weeks.

The send itself

• One clear sender name that does not change between campaigns.
• A reply-to that a human reads. noreply@ costs more than it saves.
• A plain-text part that says the same thing as the HTML.
• Link shorteners, single giant images, and RE: in a cold subject line all read as spam because they usually are.

When mail starts landing in spam

Change one thing at a time and wait. Check in this order: complaint rate, then list age, then whether the sending volume jumped, then authentication, then content. Blaming the copy first is the common mistake and rarely the cause.

What to hand back

A go or a no-go on the send, in the first line, and nothing softer than those two words. Then:

• Blockers — what makes it a no-go, each with the evidence behind it (the complaint rate, the list's origin, the missing DMARC record) and the specific correction that clears it.
• Warnings — what will not stop this send but will degrade the next one.
• What you could not check — the data you would have needed. Say so plainly rather than passing an unverified item as clear.

A go with three unstated caveats is a no-go that nobody noticed.

What not to do

Never send to a list the user cannot say the origin of — ask where it came from and stop if the answer is vague. Never make a deliverability promise, and never report inbox placement as a fact without seed-test data in front of you.