Email rendering testing is a check of how a message looks in different email clients and on different devices. Unlike the web, there is no unified rendering method, so the same template can look flawless in one client and fall apart in another. At a minimum, you need to check the display on mobile, which accounts for most opens, display with images blocked, readability in dark mode, the functionality of all links, and how the subject line appears in the message list together with the preview text. A useful habit is to send a test version to your own mailboxes with several providers and read it the way the recipient will see it. An error discovered before sending can no longer be fixed after it.
See also: Email template, Spam filters, Email subject line and preheader.
