NetDocuments has a pretty good integration with Gmail. This is the perfect solution for anyone using a Mac because it gets around the fact that NetDocuments does not have an integration with Outlook.
Once you have it installed, you will get a Save to nd button in your gmail account. This will only work in Google Chrome, so if you open up your Gmail in Internet Explorer, the Save to ND will not be there.
When you click on Save to nd, you can then browse your cabinets and choose the correct location to save your email.