Currently, between screen widths of 600-690px, there is CSS that is limiting the width of the columns to 330px. This behavior results in ugly responsive behavior for two-column layouts on many screen sizes, making any two-column layout appear unprofessional and essentially unusable.

There is also an easy fix. There is a CSS @media screen rule that just needs to be changed from "max-width: 599px" to "max-width:690" to solve the problem. I would do it myself, but there is no way to access the underlying CSS of emails sent.