Some other email clients (e.g. outlook) highlight the last read email in the email folder (inbox, etc.) by changing slightly the background shade or colour to make it stand out.

This is extremely useful when working through emails, clearing out etc. When you enter an email then leave again it shows you where you were in the list. In the case where you view an email and delete it from within the email and so return to the list the focus should go to the next (oldest) email.

This is also useful in search where you are looking for specific contents of an email from a list of matches for the search term - if you go into one email and it's not what you were looking for then it's hard to remember where you had got to in the list unless the last read email is highlighted.

I'm sure this would be very simple to implement.