You need to understand that responsive emails, while possible, can't work on every mail client. As an example, Gmail strips all your head tag from the email, so no media queries are allowed, therefore no responsiveness. From what I've tested, responsive emails can be displayed in Outlook, Apple Mail, and a few others with standard media queries. For those, you'd have to use the typical breakpoints and apply them to trs or tds. Now, that can be tricky. You have to make sure it won't break your table layout so you really need to plan in advance what will change in your layout.
If you want it to work mostly on everything, I'd suggest you use fluid layouts using % widths. But if you really want some web responsiveness, it's the same as any responsive website. Just be aware that it will not work everywhere. Like this:
@media (max-width:680px) {
.hide { display:none; }
.main { width:440px }
.header { width:440px; }
.header-img { width:440px }
.footer { width:440px; }
.footer-size { width:440px; }
}
@media (max-width:440px) {
.hide { display:none; }
.main { width:100% }
.header { width:100%; }
.header-img { width:100%; height:auto; }
.logo-img { width:75px; height:30px; }
.icon-img { width:19px; height:18px; }
.icon-wrap { width:19px; }
.footer { display:none !important; }
.footer-size { width:100% }
}
@media (max-width:240px) {
.hide { display:none; }
.main { width:100% }
.header { width:100%; }
.header-img { width:100%; height:auto; }
.logo-img { width:75px; height:30px; }
.icon-img { width:19px; height:18px; }
.icon-wrap { width:19px; }
.button { width:100%; height:auto; }
.footer { display:none !important; }
.footer-size { width:100% }
}
(That's just some code from an email campaign I worked on, btw)