Users need to be able to manually move/set the separation bar between mailbox contents and reading pane.

Most monitors are SO large today that having the reading pane occupy 70% or more of the pane is a detriment to easy navigation. The mailbox contents list is of limited use when it's needllessly narrow and many emails are of such low value that being able to see them as 33-50% of the left of the screen leaves LOTS of space for reading while being able to preview one-liners or alerts, simple yes/no replies, etc.