Email Deliverability: How to Stay Out of the Spam Folder

For most of email’s history, deliverability guidance was a folk tradition: avoid certain words, do not use too many images, warm up slowly. In February 2024 that changed. Google and Yahoo published explicit, testable requirements for bulk senders and began enforcing them. Deliverability is now a specification you can read, satisfy, and verify, which is a considerable improvement on guesswork.

The published requirements

Google defines a bulk sender as one sending roughly 5,000 or more messages a day to Gmail addresses. Yahoo applies equivalent expectations. Between the two published guidance documents, the requirements are:

The 0.3% number, and why it is the one to watch

Of all the requirements, the complaint rate is the only one you can satisfy today and fail next month, because it depends on behavior rather than configuration. Three complaints per thousand delivered messages is the ceiling. That is a small enough number that a single ill-judged send to a dormant segment can breach it, and it is measured against inbox-delivered mail, so a shrinking inbox rate makes the ratio worse just as you can least afford it.

Practical consequence: watch it weekly in Google Postmaster Tools, not monthly in your platform’s dashboard. And treat "reduce complaints" as a list problem rather than a copy problem. The people most likely to report you are the ones who forgot they subscribed.

Reputation is behavioral and it is domain-level

Mailbox providers score your sending domain and IP on how recipients react: opens and clicks help, complaints, hard bounces, and spam trap hits hurt. Two structural points follow.

• Send from a subdomain, not the root. Using something like mail.yourdomain.com for marketing keeps a bad campaign from damaging the deliverability of the invoices and replies going out from your primary domain.
• Warm up new sending domains over weeks, starting with your most engaged contacts and increasing volume gradually. A new domain sending 20,000 messages on day one looks exactly like a compromised domain, because that is what compromised domains do.
• Send on a consistent cadence. Long silences followed by large blasts are a recognizable pattern, and it is not a flattering one.
• Never re-add unsubscribes, and process hard bounces immediately rather than at the end of a campaign.

List hygiene is deliverability work, not tidiness

Removing subscribers feels like destroying an asset. It is closer to removing ballast. Dormant addresses do not open, which drags your engagement signals down, and a proportion of long-abandoned addresses are recycled by providers into spam traps specifically to identify senders who do not maintain their lists. Run a sunset policy: one re-engagement attempt at 90 to 180 days of inactivity, then suppression.

Content still matters, for narrower reasons than folklore suggests

Send multipart messages with a plain-text alternative. Keep a real text-to-image ratio, since an email that is one large image with no text is both a filtering signal and unreadable when images are blocked. Make the unsubscribe link obvious rather than 8-point grey, because a hard-to-find unsubscribe converts into a spam complaint, and a complaint costs you thirty times what an unsubscribe does under the 0.3% rule.

Instrument it before you need it

Set up Google Postmaster Tools for your sending domain, publish a DMARC record with a reporting address so you receive aggregate reports, and check both on a schedule. Seed testing tools that show inbox versus spam placement across providers are useful for diagnosis but should not replace the provider’s own data. If placement drops, stop sending, find the cause, and clean the list before sending more. Continuing to send into a reputation problem is how a recoverable problem becomes a domain migration.

Valter Brandt

Chief Marketing Officer

Valter Brandt is the Chief Marketing Officer of ThisCom, working with clients across the United States and Europe. He has led marketing strategy through the major shifts in social advertising, mobile, content marketing, programmatic media, and marketing automation.

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