Email designers waste hours testing how their templates render across different email clients and devices. Current responsive viewers only handle web browsers.
Build an email-specific previewer that shows how designs look in Gmail, Outlook, Apple Mail etc. Could integrate with popular email service providers via API.
Sell to marketing agencies and in-house teams through monthly subscriptions. Volume discounts for large teams.
Start with basic client previews using known rendering engines. Expand to include testing on actual devices through a device cloud later.
Major risk is maintaining accurate renderers as email clients update. Would need constant testing and updates.