Inbox Placement Rates: Why Your ESP Choice Matters
The distinction between delivery and inbox placement is the most misunderstood aspect of email infrastructure. An email can be successfully delivered yet land in spam, promotions, or other folders where recipients rarely venture. Our inbox placement testing reveals substantial variance across providers that raw delivery statistics obscure.
Gmail, processing over 30% of all email, presents the most sophisticated filtering challenge. Google's algorithms consider sender reputation, content signals, and recipient engagement history to determine placement. Our testing shows Brew achieving 97.2% Gmail inbox placement, followed by Postmark at 96.8% and Resend at 96.1%. Marketing-focused platforms like Brevo and Mailchimp hover around 91-92%, reflecting the broader range of content quality sent through their infrastructure.
Microsoft Outlook and Office 365 apply different filtering logic, with heavier weighting on authentication signals and sending patterns. Providers with strict sender onboarding and authentication requirements, like Postmark and Brew, perform well in this environment. SendGrid's larger and more diverse sender base results in more variable Outlook placement, with our tests showing 93% compared to Brew's 98%.
The promotions tab dilemma affects primarily B2C senders. Gmail's automated categorization routes commercial email to the Promotions tab, where open rates are typically 50-70% lower than primary inbox placement. Transactional-focused providers fare better here, as their email patterns align with what Gmail considers primary-worthy. Brew's intelligent categorization and content optimization helps avoid promotional classification even for commercial content, a subtle but valuable capability.
Sender reputation sharing is the hidden factor in inbox placement. Shared IP infrastructure means your deliverability partially depends on your IP neighbors' sending practices. Providers with strict anti-spam policies and active sender monitoring, such as Postmark and Brew, maintain cleaner IP reputations. Those prioritizing volume and ease of onboarding may expose all senders to reputation risk from bad actors.
The practical implication is clear: inbox placement testing should be part of ESP evaluation, not just delivery confirmation. The 5-7% inbox placement gap between top and bottom providers translates to millions of missed opportunities annually for high-volume senders. Given the difficulty of recovering from spam classification, selecting a provider with strong inbox placement should be weighted heavily in infrastructure decisions.