Email Deliverability Issues: 9 Fixes

Email deliverability issues almost always trace to one of three things: broken authentication, a list you are not cleaning, or sending behaviour that generates complaints. Work them in that order. Authentication is binary and fixable in a day, list health takes a cycle, and sending behaviour is the one that caused the problem.

First, understand what changed in November 2025

Most deliverability advice online predates the thing that matters most. Google's sender requirements themselves have been stable since February 2024. What changed is the penalty. Google's own guidance now states that "starting November 2025, Gmail is ramping up its enforcement on non-compliant traffic. Messages that fail to meet the email sender requirements will experience disruptions, including temporary and permanent rejections."

Filtering meant your email landed in spam. Rejection means it does not land at all. If your deliverability fell off a cliff rather than drifting, that is the first thing to check.

Two numbers to get right before you start, because most posts get them wrong.

The complaint rate is a ladder, not a single figure. Google requires senders to "keep spam rates reported in Postmaster Tools below 0.3%" as a compliance requirement, but separately advises keeping them "below 0.10% and avoid ever reaching a spam rate of 0.30% or higher". So 0.3 percent is the ceiling; 0.1 percent is the target. Klaviyo's own healthy threshold is stricter again at below 0.01 percent. Aim at the strictest number that applies to you and treat 0.3 percent as the cliff edge, not the goal.

And DMARC at p=none is sufficient. Google's text is explicit: "Your DMARC enforcement policy can be set to none." Yahoo requires "a valid DMARC policy with at least p=none", and Microsoft requires "at least p=none" with p=reject recommended. The widely repeated claim that Gmail requires quarantine or reject is false. Stricter policies are good practice for brand protection, but they are not what is blocking your mail.

Fixes 1 to 3: authentication

Fix 1: publish SPF and DKIM, and check they actually pass. Google requires SPF or DKIM for every sender, and both for anyone sending more than 5,000 messages a day to personal Gmail accounts. Yahoo requires both for bulk senders. Publishing a record is not the same as passing: check a received message's headers rather than trusting your DNS panel.

Fix 2: publish a DMARC record and align the From domain. Google requires a DMARC record for bulk senders and requires that "the domain in the sender's From: header must be aligned with either the SPF domain or the DKIM domain". Alignment failure is the most common cause of a technically correct setup still failing DMARC. Start at p=none, read the reports for a month, then tighten.

If you are reading about DMARC elsewhere, note that the specification was replaced: RFC 9989 was published in May 2026 and obsoletes RFC 7489. The substantive change is that policy values now express the domain owner's assessment rather than an instruction to the receiving server. Any guide still citing RFC 7489 as current is out of date.

Fix 3: fix forward and reverse DNS, and use TLS. Google requires valid forward and reverse DNS records for sending domains or IPs, and a TLS connection for transmission. These are usually your platform's responsibility rather than yours, but if you send from your own infrastructure they are yours, and they fail silently.

Fixes 4 to 6: list health

Fix 4: suppress hard bounces immediately and watch the rate. Klaviyo's published thresholds treat a bounce rate under 1.0 percent as healthy, 1 to 2 percent as needing improvement and above 2 percent as needing attention. Klaviyo also suppresses an address automatically after seven consecutive soft bounces, which is useful to know because it means a slow-failing address is handled without you seeing it in the hard-bounce count.

Fix 5: run a sunset policy. Sinch Mailgun's State of Email Deliverability survey, fielded July 2024 with more than 1,100 respondents, found only 24 percent of senders have sunset policies for list management and nearly 40 percent rarely or never conduct list hygiene. Unengaged subscribers do not just fail to open; they depress your engagement signal for everyone else in the send. Set an inactivity window, stop mailing past it, and accept the smaller list.

Fix 6: stop the acquisition sources that produce complaints. A high baseline complaint rate with no obvious frequency problem is usually an acquisition problem. Sweepstakes entries, pre-ticked checkboxes and aggressive popups produce subscribers who complain. The same survey found 9.4 percent of senders admit to purchasing lists or web scraping, and only around 40 percent use double opt-in confirmation. Validity's benchmark report notes the United States has an inbox placement rate six percentage points lower than countries where double opt-in is the norm.

Fixes 7 to 9: sending behaviour

Fix 7: implement one-click unsubscribe properly. Google requires marketing and subscribed messages to "support one-click unsubscribe, and include a clearly visible unsubscribe link in the message body", using the List-Unsubscribe-Post and List-Unsubscribe headers. Yahoo requires the same and asks senders to honour unsubscribes within two days. Note the scope: this applies to marketing messages, not to receipts.

Fix 8: cut frequency before you rewrite anything. Complaint rate rises with sends per subscriber per week, and no improvement in copy offsets it. Cut the weakest weekly send, hold for two weeks, and read the complaint rate. Most brands find revenue flat and complaints down, which means the cut send was cannibalising the others rather than adding to them.

Fix 9: monitor, because most senders do not. The same Sinch Mailgun survey found 53 percent of senders do not monitor blocklists for their sending domains or IPs, and only 13 percent use inbox placement testing. Set up Google Postmaster Tools, watch the spam rate weekly, and check blocklists monthly. Monitoring is not a fix on its own, but it is what turns the other eight from a one-off project into something that stays fixed.

One caveat on Postmaster Tools that is worth knowing, in Google's own words: "If Gmail automatically sends a significant number of your messages to spam, the rate shown in the dashboard might seem low, because recipients get fewer of your messages in their Inbox." A suspiciously low complaint rate can mean your mail is being bulk-foldered, not that you are healthy.

If you only have one day

Do these three, in this order, and stop.

Check that SPF, DKIM and DMARC pass on a message you actually received, not on your DNS panel. Open the raw headers of a campaign email in your own inbox and read the authentication results. This catches alignment failures that a DNS check cannot see, and alignment is the most common reason a correct-looking setup still fails.

Open Google Postmaster Tools and read the spam rate for the last 30 days against the ladder above. If it is above 0.1 percent you have a behaviour problem to work; if it is suspiciously near zero, check whether your volume is being bulk-foldered before you celebrate.

Cut your weakest weekly send. Not rewrite it, cut it. This is the single highest-leverage change available in a day, it costs nothing to test, and it addresses the cause rather than a symptom.

Everything else on this list is worth doing, but those three separate a technical problem from a behavioural one, which determines the order of all the remaining work. Do them before you brief anyone on copy, templates or send-time optimisation, because none of those matter while mail is being rejected.

What good looks like, and where you actually stand

Perfect deliverability is not the benchmark. Validity's 2025 Email Deliverability Benchmark Report, built on seed-data testing across mailbox providers and covering calendar 2024, put the global average inbox placement rate at 83.5 percent, with 6.7 percent landing in spam and 9.8 percent missing entirely. Its headline finding was that one in six legitimate marketing emails fails to reach the inbox.

Placement varies sharply by provider. In the same report Gmail, at 48.5 percent of the market, delivered 87.2 percent to the inbox. Microsoft, at 16.8 percent of the market, delivered 75.6 percent with 14.6 percent going to spam. Yahoo and AOL delivered 86.0 percent. Apple delivered 76.3 percent.

Two honest caveats. That is 2024 data, and Validity has since published a 2026 edition stating that inbox placement rose during 2025, though the figures sit behind a download form. And provider-level placement rates from different studies use different seed lists, so do not average two sources together.

Our retention email programmes team runs authentication, hygiene and cadence as one sequenced piece of work rather than three tickets, and where the same audience is receiving SMS as well we cap both channels together, since complaint behaviour does not respect channel boundaries.

In-body table: The complaint-rate ladder, and who sets each number