Responsive content/format with Bulk Mail sender?

[JTrollerb123]

So I've sent an email using the bulk mail sender - And the text looks good on laptop based email client like Gmail ... but when viewing it on my iPhone, the text is way too tiny and has to be pinched to zoom in and then scroll.  What guidelines / templates / best practices are in place to ensure emails sent are legible (without zooming in) on both computers and mobile devices?

Thanks,