While it's a decent workaround, I wouldn't call it ideal, namely because it requires a second window open when you might be starved for screen real estate and also because accidentally opening your mail once in a normal window could cause damage.
The Ghost Incognito extension allows you to force certain sites to always use Incognito mode. I realize using an extension to accomplish this objective is somewhat ironic, but seems like it'd work in this case if you can trust that one extension.
