Often you get a list of list of unread emails. You start at the top and work down. What is a pain is to view the mail, delete it, get sent back to the index, click on the next message on the list, and repeat.
What is needed on the view message screen is the ability to delete the current message and view either the previous or next message in a single step.
The delete button is there. The view previous/next buttons are there. Just put them together with a 2-in-1 action please.
Squirrelmail has a plugin for this and it's a great feature. (FYI the Squirrelmail version is buggy)