Most eCommerce brands confuse email deliverability with delivery rate. Delivery rate only confirms the email reached the mail server. Deliverability is about where it lands: primary inbox, promotions tab, or spam. A 99% delivery rate means nothing if campaigns are invisible in spam folders. This guide covers how inbox provider standards have shifted in 2026, the most common mistakes brands make, and a step-by-step setup for authentication, domain warmup, list segmentation, and ongoing monitoring.