A Practical Guide to Email Deliverability

An email platform reporting a message as “delivered” is telling you almost nothing useful. Delivered means the receiving server accepted it without bouncing, and it says absolutely nothing about whether the message landed in an inbox anyone will see or in a spam folder nobody checks. That gap, between technically delivered and seen, is where most people’s understanding of deliverability quietly goes wrong.

The real metric is inbox placement, and it’s decided mostly by one thing: how much a mailbox provider like Gmail or Outlook trusts the sender.

Authentication is where trust starts

Before reputation even enters the picture, a message needs to be technically verifiable as coming from who it claims to. Three records do this work. SPF specifies which servers are allowed to send mail for a domain. DKIM cryptographically signs outgoing mail so it can’t be forged in transit. DMARC tells receiving providers what to do with mail that fails those checks, and reports back on what’s happening.

This stopped being a nice-to-have in 2024, when Gmail and Yahoo began requiring SPF, DKIM and DMARC for anyone sending bulk mail, with noncompliant senders facing outright rejection or automatic spam folder placement. If a sender does exactly one thing to improve deliverability, authenticating the domain properly is it.

Reputation is a running scorecard

Once authentication confirms a message is from who it claims, mailbox providers score the sender on an ongoing basis, and that score, more than anything else, decides where future mail lands. Engagement carries the most weight: opens, clicks, replies and mail getting pulled out of spam all signal that recipients want what’s arriving. Complaint rates matter enormously too, since someone marking a message as spam is about the most damaging signal a sender can generate, and keeping that rate well under 0.3 percent is the difference between a healthy reputation and a slow decline. Hitting spam traps, dead addresses planted specifically to catch senders with stale or purchased lists, does serious damage fast. And sending consistently, rather than in erratic spikes, especially from a newer domain, reads as more trustworthy than unpredictable bursts of volume.

The underlying logic behind all of it is simple: reputation is earned by sending mail people want, and every signal above is just a proxy for that one question.

The lever most people ignore

Because engagement drives reputation so directly, continuing to email people who never open anything is actively harmful, not neutral. It drags down engagement metrics and raises the odds of hitting a spam trap hiding in an old, unmaintained list. The fix is unglamorous but effective: remove hard bounces immediately, run a re-engagement sequence for chronic non-openers and then stop emailing whoever still doesn’t respond, and never buy or scrape a list in the first place, since that’s the fastest route to traps and complaints at once. A smaller list of people who open the mail will consistently outperform a bloated one that technically has more names on it.

Diagnosing a sudden problem

When open rates drop sharply or mail starts landing in spam, work through the likely causes in order: confirm SPF, DKIM and DMARC are still correctly configured, since a small DNS change can quietly break authentication. Check domain reputation directly through a tool like Google Postmaster Tools rather than guessing. Audit the list for chronic non-openers that should have been suppressed already. Review anything that recently changed, a new sending domain, a sudden volume spike, a shift in content. And if the damage is already done, pull sending back to only the most engaged subscribers for a while to rebuild trust before scaling volume back up.

Deliverability rarely announces itself. It just quietly determines whether every other decision in an email program, the subject lines, the segmentation, the offer, ever reaches anyone at all.