MailChimp templates are all 600px wide, as a comparison, and that's what they recommend. It's annoying, since so many people have large monitors (notably me - so much white space around my drafts), BUT as you point out so many recipients are viewing emails on mobile devices (phone, tablet, even netbook-size laptops) as well as webmail clients and the Outlook preview pane.
My Mail window is at most 700px wide. That’s about 400px to 500px for the content. Most of the text-only mails I receive fit comfortable in that space without any scrolling and the line length is just perfect (60 to 80 characters per line).
Why would I want my Mail window to be any bigger? I have all the space I need for comfortable reading and filing and more than half a screen to do something useful with. I don’t really see how the size and resolution of the monitor figures into the size of the window of your mail app at all.
Yes, displaying a list with as many messages as possible and as many mailboxes as possible for comfortable filing might require additional space (beyond the 700px I like) but that seems to be inconsequential for the size with which you display the actual email message (my 400px to 500px), i.e. you don’t have to maximize the window of your mail app to achieve that.
Anyway, why use fixed width layouts at all? Are mail apps so backwards that fluid layouts are not possible?
In reality, you can't trust all browsers to behave the way standards dictate. If you're doing business online, through email, etc. you need to be able to reach as many people as possible and that may mean sending 600px wide emails instead of 900px.
And in case my original point lacked context, I was talking about marketing for a business to customers - even B2B has to assume a variety of customers that have little to no technical skills and aren't opening emails on their 19" laptop.
