First, I'd like to thank you for your feedback on this! Our team has been hard at work taking steps to make our users emails look better and work better for users that are viewing them on mobile devices. With our current generation editing experience, we have introduced a large number of templates that are specifically optimized for mobile viewing, so that we do not have to pass on the requirement to build media queries. You can identify these templates when you're creating a new campaign by looking for the mobile icon shown here:
In addition, our new third-generation editing experience that is currently in beta testing has mobile responsiveness built in from the start, so that all emails in any template will look great on a desktop/notebook and mobile devices.
If you'd still prefer to build an email using media queries that you've personally designed, the best option that's available would be to create a custom code email where you can define your own CSS and HTML without restrictions to give you complete control over the entire email.
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