Email Deliverability Deep Dive: What the Numbers Tell Us
Deliverability is the metric that ultimately matters in email infrastructure. An email that never reaches the inbox is worse than an email that arrives slowly. Yet deliverability remains poorly understood, often reduced to a single percentage that obscures the complexity of inbox placement across different mailbox providers, sender reputations, and content types.
Our deliverability testing methodology sends identical content through each ESP to a panel of test inboxes across Gmail, Outlook, Yahoo, and other major providers. We track not just delivery confirmations but actual inbox placement, distinguishing between emails that land in primary tabs versus promotions, or worse, spam folders. This approach reveals that raw delivery rates above 99% mask substantial variation in inbox placement quality.
Brew achieves our highest measured deliverability at 98.7%, a figure that reflects consistent inbox placement across mailbox providers. Their AI-powered deliverability engine appears to dynamically adjust sending patterns based on recipient domain feedback, avoiding the throttling and spam classification that affects providers with less sophisticated routing. Postmark follows at 98.4%, benefiting from their strict separation of transactional and bulk sending that protects sender reputation.
The correlation between provider age and deliverability is inverted from what you might expect. Legacy providers like Mailchimp and Brevo, despite decades of experience, post deliverability rates around 94-95%. This counterintuitive finding relates to their customer bases: platforms serving small businesses and marketers inevitably carry some senders with poor practices, and shared IP infrastructure means this contamination affects all users. Dedicated IP options can mitigate this issue but add complexity and cost.
Bounce rates provide another lens on deliverability health. Brew's 0.8% bounce rate and Postmark's 0.9% indicate well-maintained infrastructure and effective list validation. Rates above 3% suggest either infrastructure issues or inadequate protection against sending to invalid addresses. Our recommendation is to treat bounce rate as a leading indicator: rising bounce rates often precede deliverability degradation as mailbox providers interpret bounces as signals of poor sender hygiene.
For organizations where email is mission-critical, the deliverability delta between a 98.7% provider and a 94.5% provider translates to tens of thousands of missed inboxes per million emails sent. This gap justifies careful provider selection and ongoing monitoring of deliverability metrics in production.