Email Deliverability: Why Your Mail Lands in Spam or Bounces, and the Order to Fix It

Email deliverability is the likelihood that a message you send arrives in the recipient's inbox rather than being rejected or filed in spam. It is not the same as the delivery rate your sending platform reports, which only counts messages the receiving server accepted. A message can be accepted and still land in a folder nobody opens.

That split is the first diagnostic fork. A rejected message produces a bounce with a code and a reason, so the evidence is in your logs. A message filed in spam produces nothing: no error, no alert, only an open rate that falls or replies that stop. Treat the two as separate investigations, because the fixes barely overlap. Bounces point at addresses, DNS and technical policy. Spam placement points at reputation, which is the receiving provider's running judgement of how recipients treat your mail.

Start with evidence, then work from the cheapest, most binary checks to the most subjective. Authentication either passes or fails, so it goes first. Complaint and bounce rates are numbers you can read, so they go second. List quality takes an audit, and content is last because it is the hardest to prove and the most tempting to fiddle with.

We recommend running the sequence below on one sending domain at a time. If three tools send as your domain, isolate the one that is failing; changing all of them at once makes the result unreadable.

A hard bounce is a permanent rejection, such as an address that does not exist. A soft bounce is temporary, such as a full mailbox or a server that throttled you. Both come with an SMTP response, and the text after the code usually names the cause: unknown user, authentication failure, policy block or rate limit. Group your bounces by that reason text before you read anything else, because one category often dominates.

A spam-folder problem leaves no log line. The usable signals are indirect: opens falling at one mailbox provider while holding at others, a seed address at that provider showing the message in spam, and the provider's own dashboard moving. If the drop is confined to Gmail addresses, look at Gmail's data. If it is confined to a single campaign, look at that campaign's list and content. If it began the day a new tool started sending as your domain, look at authentication for that tool.

Google's sender guidelines require every sender to authenticate with SPF or DKIM, to have valid forward and reverse DNS for sending IPs, and to keep spam rates low. Anyone sending more than 5,000 messages a day to Gmail accounts falls under Google's stricter tier and must authenticate with SPF, DKIM and DMARC, make the From domain align with SPF or DKIM, and support one-click unsubscribe in marketing and subscribed mail. Yahoo's published requirements for bulk senders follow the same pattern, and Yahoo asks that unsubscribe requests be honoured within two days.

The practical test is a received message, not a DNS lookup. Send a test to a mailbox you control, open the full headers, and read the authentication results: SPF pass, DKIM pass, DMARC pass, and the domain in the visible From address matching the domain that passed. A record can exist and still fail alignment because a third-party tool signs with its own domain. That specific failure is common after someone adds a new platform to the marketing stack. Our companion playbook on authentication covers the record syntax; here the point is only to confirm pass or fail before looking elsewhere.

Reputation is the mailbox provider's running estimate of whether recipients want your mail. Complaints and bounces are the signals most often discussed, and Google's guidelines tie reputation to your domain and sending IP, but neither Google nor Yahoo publishes the full scoring formula, so any guide that claims to know the weights is guessing. What they do publish is thresholds. Google asks senders to keep user-reported spam rates in Postmaster Tools below 0.10% and to avoid ever reaching 0.30%; Yahoo states its limit as 0.3%.

Recovery is slower than damage. The mechanism is simple: send less, to people who recently engaged, with an easy way out, and let the provider's data catch up. In practice that means suppressing every address that has not opened or clicked in a period that suits your sending cadence, mailing the engaged segment only, and widening back out in steps while watching the spam-rate line. Illustrative example: a list of 40,000 with 6,000 recent engagers. Mail the 6,000 for two or three weeks, then add the next most recent 6,000, stopping to hold position if the spam rate climbs. This is slower than a blast, and that is the point.

List quality explains more spam-folder problems than wording does. Purchased, scraped or rented lists, pre-ticked consent boxes and old addresses all generate complaints and bounces from people who never asked for the mail. Yahoo advises against buying lists and against auto-checked opt-in boxes, and recommends honouring the frequency subscribers signed up for: a monthly newsletter subscriber who starts receiving daily mail will complain. If you use double opt-in, a bounced confirmation also removes a bad address before it can hurt you.

Content comes after list and cadence because it is rarely the root cause. Check for what changed: a new link domain, a link shortener, a misleading subject line, or a missing physical address. Providers do not publish content rules in detail, so treat these as things to rule out, not confirmed triggers. The FTC's CAN-SPAM guidance sets the legal floor in the United States: accurate header and subject information, a clear opt-out that works, a valid postal address, and honouring opt-outs promptly. These are also behaviours that keep complaints down. This post is not legal advice; check requirements for each country you send to.

Postmaster Tools is Google's dashboard for mail you send to personal Gmail accounts. Its dashboards cover spam rate, reputation, message authentication and delivery errors. You verify your sending domain there, then read the charts. Its limits matter: it reports only on Gmail traffic, it may omit data on low-volume days (Google does this to protect user privacy), and it describes the symptom, not the cause. A rising spam-rate line tells you to look at the list or the campaign that started when it rose.

Illustrative reading of a chart: the spam-rate line sits near zero for weeks, then rises the same day a new import of event attendees was mailed. Nothing about the content changed. The fix is to suppress that segment, mail it only after a re-permission message, and watch the line fall back over the following days. Without the change log you would have rewritten the template, which was never the problem.

Pair it with your own weekly routine: hard-bounce share by reason, complaint rate by campaign, opens by mailbox provider, and a short log of every change made to templates, tools or lists. When something breaks, that change log is often the whole diagnosis. Who this is not for: a sender mailing a few hundred people a month rarely produces enough Gmail volume for Postmaster Tools to chart anything, and should rely on bounce reasons, header checks and plain list hygiene instead.