Currently, you will need to use outside services to render email previews for various email clients. We recommend trying litmus.com to check out the previews of your email newsletters for different clients. You can send a test mail to this service and it lets the mail render on various email clients, captures the output, and then displays the previews.
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Mail Designer 365 does provide a preview for mobile devices. Keep in mind that this is only for the sole purpose of providing a rough idea of how the newsletter may look like on mobile devices. The actual rendering may be different, depending on the email client.
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Please visit this page for further information on how to preview your design for different email clients:
Checking your design on different email clients guide.