Diagnosing a deliverability problem

Opens fell off a cliff. Work down this list in order — the answer is almost always found in the first three steps.

1. Segment by mailbox provider

Before anything else, split the metrics by receiving domain: Gmail, Outlook/Hotmail, Yahoo, corporate domains.

A drop concentrated at one provider is a reputation problem with that provider. A drop across all of them at once is usually a content, list or authentication problem — or a tracking problem.

This single step tells you which branch of the tree you are on. Skipping it is why deliverability investigations go in circles.

2. Rule out measurement

Did opens fall, or did open tracking fall? Apple Mail Privacy Protection inflated opens when it arrived; anything changing the share of Apple Mail readers moves the number without anything real happening.

Check clicks. If clicks held while opens fell, you have a measurement change, not a delivery problem.

3. Check Postmaster Tools

For Gmail, Google Postmaster Tools shows your domain reputation, IP reputation, authentication pass rates and — critically — your spam complaint rate.

Gmail's threshold for bulk senders is low, and exceeding it is the most common cause of a sudden Gmail collapse. This is the single most useful diagnostic available and it is free.

4. Check authentication

Did SPF, DKIM or DMARC start failing? Common triggers: a new sending tool pushing SPF past ten lookups, a DNS change, a domain migration, an ESP rotating keys.

DMARC reports will show this.

5. Check what you sent

• Did you email a segment you had not mailed in months?
• Did volume spike?
• Did you change the from-name, from-address or sending domain?
• Did you import a new list?

Any of these can trigger a reputation reassessment. A new sending domain has no reputation at all and must be warmed gradually.

6. Check blocklists

Less common than people assume, but check. If you are listed, find the cause before requesting delisting — being delisted while still doing the thing achieves nothing.

7. Seed testing

Send to a set of seed accounts across providers to see actual inbox placement. This tells you what your ESP's delivery rate cannot.

What trips people up

• Averaging across providers and hiding the real story.
• Requesting delisting without fixing the cause.
• Assuming spam-folder placement when the real change was open tracking.

You have got this when

Your first move is to segment by provider, and your second is to open Postmaster Tools.