I'm hurt that you think I would give you a bad solution :-)
If you tap on the Contexts button in the OmniFocus for iPad sidebar, you get all of your contexts. If you tap the > button, you can drill down through the hierarchy in the sidebar, and you would see your Email context, which would also have a > button (as it has contexts nested inside). While viewing the Email context along with its children, you can use the disclosure triangles at the upper right hand corner of each context to open or close that context. If you tap and hold on one of the triangles, you'll get the option to collapse or expand all of them in the view. So, you can show only the ones you want at the moment, and hide the rest. I do a similar construct for my computer activities, dividing along the "internet"/"no internet" boundary, then dividing by device type and specific device. This gives me contexts such as "Internet : Mac: iMac i7", "Internet : iPad : iPad 2", "Mac : MacBook : Snow Leopard". I can easily pick up an iPad, launch OmniFocus, and look at just the actions available for that iPad, or that iPad plus general internet tasks but not internet tasks related to any of the Macs, etc. Similarly, if I'm on the MacBook Air somewhere without an internet connection, I can look at tasks to be done on that Mac without internet, or tasks to be done on any Mac.