Following on from yesterday's post, I've been trying out Postbox. Previously I tried Sparrow, which was OK, but over-simplified to the extent that it is hard to tell what processes have occurred. The point of needing an new email client is to feel comfortable and confident in the medium, and Sparrow failed to achieve that for me.
So far, Postbox is looking good, like a vastly improved Outlook. Improved by being much more customizable, up to date and generally rather good (such as reminding you when you forget to actually attach those attachments you mentioned, message templates - which will be really useful for responding to students, etc). I'm currently on the 30 day free trial, but I anticipate buying the full version (price recently reduced to $9.95) soon.
Give it a try.