Width Adjustment of Mailbox List vs Reading Pane
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.

-
WENDY WHITTALL commented
I'd agree with this option, but for the opposite reason... My iPad screen has limited viewing space and the addition of that stupid ad strip running along the whole side of every email leaves only a ridiculous 3 inch viewing window. Not everyone uses a huge computer monitor and having the ability to adjust segments to work for your particular screen would be helpful.