The ideal width of marketing emails is 600px, does it also apply to enterprise product emails? or it can be responsive without any max width limit?
Responsive design is preferable, making a set hard coded width undesirable. Far more people view web content, including emails, on mobile devices today. Unlike 10-15 years ago where you could be certain every email was seen on a desktop/laptop, today there's really no telling if the viewing deice is 320px wide, 700+px wide or 1000+px wide.
The only real thorn is Outlook or luddites still using circa 2001 email clients. There should be some attention to fallbacks, but overall there comes a point where you simply can't support things 15 years old.
Truth is though, even with responsive design things aren't that bad for Outlook users provide you try and avoid the more "cutting edge" CSS3 features out there, or plan ahead for them failing. (Don't use @font-face, CSS shadows, transformations, filters, etc)