Is This a Delivery Problem? How to Tell Before You Spend Money

When email results drop, "deliverability" is usually the first explanation offered. Sometimes that is right. Often the mail is still arriving, and something else changed: the list, the offer, or the way results are counted. Those problems have very different fixes and very different costs, so an hour of checking is worth doing before you hire anyone or move to another platform.

This article is for you if you run email for a business and don't work with the protocols day to day. It only uses evidence you can reach yourself: your sending platform's reports, Google Postmaster Tools, a seed test and a free domain check. The principle underneath all of it, that mailbox providers deliver the mail their users want and expect, is covered in Foundations of Email Deliverability.

No check here, and no one you hire, can promise where your mail will land. Google says it does not accept allowlist requests from email providers and "can't guarantee messages sent by email providers will pass Gmail's spam filters" (Gmail sender guidelines). What you can do is find out which kind of problem you have.

Four problems that look alike

These are not sealed boxes. A list problem left alone becomes a delivery problem: Google says that if messages from your domain are frequently reported as spam, future messages are more likely to be marked as spam, and "over time, user spam reports can lower your domain's reputation" (Gmail sender guidelines). That is why the list checks below come before any spending.

Don't judge delivery on opens

Apple's Mail Privacy Protection downloads an email's remote content "in the background by default — regardless of whether you engage with the email" (Apple). Senders learn when a message was opened through that same remote content, so for recipients who use the feature, a recorded open no longer shows that anyone read the message.

An open rate can therefore change without any change in where your mail landed. Judge delivery on bounces, complaints and the placement evidence below, and judge interest on clicks and sales. Tracking and Measurement Distortion covers the other sources of skew.

The evidence you can gather yourself

1. Your sending platform's bounce report

A bounce is a receiving server saying no. The code tells you whether it's temporary or permanent: replies starting with 4 are transient failures and replies starting with 5 are permanent (RFC 5321 §4.2.1), and the enhanced codes follow the same split, 4.x.x and 5.x.x (RFC 3463).

Read the text of the bounces, not only the count. M3AAWG notes that the codes alone "do not clearly state whether or not the address should be removed", and that the descriptive text can point to a problem with sending infrastructure or content, or show that the sending IP is on a blocklist (M3AAWG Senders BCP).

• Mostly "user unknown" or "mailbox does not exist": a list problem. M3AAWG says large volumes of hard bounces "are too often indicative of a poorly managed registration process or an old or misapplied mailing list".
• Text naming authentication, policy, reputation or a blocklist, concentrated at one provider: a delivery problem at that provider. For example, Gmail may reject unauthenticated messages with a 5.7.26 error, and typically answers with 4.7.28 when you exceed its sending-rate limits (Gmail sender guidelines).

Delivery Troubleshooting Playbooks walk through bounce text by provider.

2. Your complaint numbers

Complaints reach your platform through feedback loops, which mailbox providers offer to forward spam reports back to the sender (RFC 6449). Yahoo sends a copy of the complaint when a user clicks "report spam" (Yahoo). Gmail shows a spam rate in Postmaster Tools instead.

The providers publish their limits:

• Gmail: keep the spam rate below 0.1% and never let it reach 0.3%. Google says rates above 0.1% already have a negative impact on inbox delivery for bulk senders (Gmail sender FAQ).
• Yahoo: keep the spam rate below 0.3%, calculated on mail delivered to the inbox (Yahoo).

Rising complaints usually mean a list or consent problem that is starting to show up as a delivery problem. Gmail's guidance is direct: people who didn't sign up to get your messages "might mark your messages as spam" (Gmail sender guidelines).

3. Google Postmaster Tools

Postmaster Tools is Google's free console for senders. Its dashboards cover spam rate, reputation, message authentication and delivery errors, and its data only applies to messages sent to personal Gmail accounts, the ones ending in @gmail.com or @googlemail.com (Google).

• You add the domain your mail is authenticated with (the DKIM d= domain or the SPF Return-Path domain), then verify it with a DNS record. Whoever manages your DNS will need to add that record.
• Data can be missing when the number of messages on a given day is too low. Google withholds it to protect users' privacy. An empty dashboard is not a sign of a problem.

Google Postmaster Tools explains each dashboard.

4. A seed test

A seed test sends your campaign to a set of test mailboxes at several providers and reports which folder each copy landed in. It is the only item here that shows placement directly, but only for those mailboxes.

Placement is decided per recipient as well as per sender. Google says messages from an address in the recipient's contacts are less likely to be marked as spam, and that when a recipient marks a message as not spam, future messages from that sender should reach their inbox (Gmail sender guidelines). Test mailboxes have no such history with you. Read a seed result as a pattern across providers, not as a measurement of your subscribers. Placement Measurement Methodology covers where seed results mislead.

5. The free domain check

The domain check reads the SPF, DKIM, DMARC and MX records your domain publishes in public DNS, and looks up the addresses your SPF record authorises on the blocklists it queries. It cannot see how your sending platform uses those records, your reputation at mailbox providers, or where your mail ends up. Reading a Domain Check Result explains each verdict.

Authentication is not optional at the large providers:

• Gmail requires SPF or DKIM from all senders, and SPF, DKIM and DMARC from anyone sending more than 5,000 messages a day to Gmail accounts (Gmail sender guidelines).
• Yahoo requires SPF or DKIM from all senders, and SPF, DKIM and a DMARC policy of at least p=none from bulk senders (Yahoo).
• Microsoft requires domains sending more than 5,000 emails a day to Outlook.com to comply with SPF, DKIM and DMARC. Non-compliant mail goes to the junk folder and may be rejected with 550 5.7.515 (Outlook.com postmaster policies).

Reading the evidence together

What each finding means for the decision

Fix the list first

When: permanent bounces for addresses that don't exist, rising complaints, a segment you haven't mailed in a long time, or addresses that were bought or collected without a clear opt-in.

Why first: the providers weigh what your recipients do. Gmail tells senders to send "only to people who want to get messages from you" and not to purchase addresses (Gmail sender guidelines). Yahoo says mailing people who aren't reading your messages, or who report them as spam, "will harm your delivery metrics and reputation" (Yahoo). Spamhaus treats a spam-trap hit as "a sign of poor data hygiene or issues with your marketing sign-up process", and advises fixing data collection rather than trying to locate the trap (Spamhaus, Email Deliverability 101). M3AAWG recommends removing addresses that bounce consistently over consecutive campaigns (M3AAWG Senders BCP).

Nobody outside your business can fix who is on your list. Start with List Hygiene and Sunset Policies and Consent Methods.

Get help

When: the evidence points at delivery (rejections naming reputation or policy, a sustained spam rate or delivery errors in Postmaster Tools, a blocklisting that comes back), and your list and authentication checks came back clean. Also when the evidence conflicts and you can't tell.

What to bring: samples of the bounce text, your complaint rates over time, Postmaster Tools screenshots, and your domain check result. That turns the first conversation into a diagnosis rather than a discovery exercise.

What to expect: the providers set conditions on their own help. Gmail says bulk senders with a spam rate above 0.3% are ineligible for its delivery mitigation until the rate stays below 0.3% for 7 consecutive days (Gmail sender FAQ). Microsoft says its support team may not be able to assist senders who don't comply with its policies (Outlook.com postmaster policies). Bringing complaints down comes before escalation, whoever does the work. Cross-Provider Escalation and Mitigation Channels lists where to go.

Change ESP

This is rarely the first answer. It makes sense when the problem is in what the platform controls and can't be resolved there, for example:

• It can't authenticate mail with your own domain. Google tells senders who use an email service provider to verify that the provider authenticates their domain's email with SPF and DKIM (Gmail sender guidelines).
• Shared-IP reputation keeps pulling you down. On a shared IP address, "the activity of any senders using a shared IP address affects the reputation of all senders for that shared IP address" (same source). If that recurs and the platform can't separate your mail, it's a platform problem.

A move doesn't fix a list or content problem. Gmail tracks volume, feedback and limits per domain as well as per IP address, and spam reports lower a domain's reputation (same source). Because that history is recorded against your domain and not only against the platform's addresses, the same domain carries it to a new platform. A move is also a significant change to your sending infrastructure: Google advises increasing changed traffic separately, and avoiding sudden volume spikes without a history of sending large volumes (same source).

Related

• Foundations of Email Deliverability — the wanted-and-expected principle behind every check here
• What Mailbox Providers Measure, and Who on Your Team Owns Each Part — the signals behind these findings, and who acts on each
• Reading a Domain Check Result — what each verdict on the domain check means
• The Seven-Step Delivery Model — the campaign loop, including the recipient-issues taxonomy
• Metrics and Benchmarks — the numbers in more depth