An inbox preview tool is a service that is used by email marketers before sending a campaign to see how an email renders in different email clients (for more details on rendering issues, see email rendering).
Most advanced services combine main desktop and web-based email clients with differents browsers, OS and devices (desktop, tablets, smartphones, Kindle, etc.). Tested environments are on the rise with device and mobile fragmentation. On some inbox preview tools, messages are tested on more than 50 combinations or configurations. In each country some local inbox preview services may adapt the array of combinations according to local ISP and Web-based email clients.
Inbox preview services are built into many email marketing softwares and ESP platforms. It is also possible to use standalone tools by using a test address or by uploading a copy of the email.
An example of Litmus inbox preview tool: