Email Deliverability Comparison: I Tested the Top Providers and Here’s What I Found

Email deliverability depends partly on a transactional email service provider’s infrastructure and policies, but the sender’s authentication, list quality, reputation, content, and engagement practices account for much of the outcome. Using GlockApps seed-list tests with newly created free accounts, an identical domain and template, and no domain warm-up, the comparison found that Postmark had the strongest overall inbox placement at 83.3%, followed by Mailtrap at 78.8%, Amazon SES at 77.1%, Mailgun at 71.4%, and SendGrid at 61.0%, though performance varied substantially by mailbox provider. Outlook, Hotmail, Yahoo, and AOL repeatedly presented placement difficulties, while Amazon SES performed comparatively well with Outlook and Hotmail. The findings are limited by the use of free plans and shared IP pools, whose reputations may be influenced by other users, as well as changing filters, sender histories, timing, regions, and provider configuration changes. ESPs can support deliverability through authentication, standards compliance, anti-spam enforcement, and IP reputation management, but senders must maintain sound technical setup and responsible sending practices; SMTP and API delivery methods should produce similar results when they use the same underlying sending infrastructure.

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