Email Deliverability: Getting Into the Inbox

WHY IT EXISTS: In the early internet, anyone could send email to anyone. This was quickly abused by spammers. In response, Internet Service Providers (ISPs) like Gmail and Outlook created sophisticated filtering systems to protect their users. Email deliverability is the practice of navigating these systems to ensure legitimate emails reach their intended recipients.

THE MENTAL MODEL: Think of email deliverability as a credit score for your sending domain and IP address. ISPs act like lenders, constantly evaluating your reputation. Sending relevant, wanted emails to engaged users builds your score. Sending unsolicited mail, hitting spam traps, or getting high complaint rates damages your score, causing your emails to be filtered into spam or blocked outright. Your goal is to maintain a high reputation score.

HOW IT WORKS: Deliverability is determined by hundreds of factors. Key technical elements include authentication protocols like SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting & Conformance). These prove you are who you say you are. Engagement metrics are also critical: ISPs track open rates, click rates, bounce rates, and spam complaints. High positive engagement signals that your mail is wanted.

WHEN TO USE IT: You need to actively manage deliverability any time you send automated or bulk email. This applies to transactional emails (password resets, shipping notifications), marketing newsletters, and product announcements. If your business relies on email to communicate with users, deliverability is a primary concern. Ignoring it is like building a product and not checking if customers can access it.

WHEN NOT TO USE IT: There's no scenario where you'd want bad deliverability. However, the level of effort changes. For one-to-one personal correspondence from your own inbox (e.g., Gmail to a colleague), the platform handles most of this for you. The intense focus on deliverability practices is for application-sent email at any kind of scale.

ONE CANONICAL EXAMPLE: A company buys a large email list and sends a marketing blast. Because the list is low-quality, many addresses are invalid (hard bounces) or haven't engaged in years. A few are 'spam traps'—email addresses set up by ISPs to catch spammers. The ISPs see the high bounce rate and spam trap hits, immediately flag the sender's IP and domain as suspicious, and start routing all future emails from that company—even legitimate transactional ones—directly to the spam folder. Their deliverability is now ruined.