No.  Unlike IMP, we have to factor in the scrollbar that will appear 
for longer message lists.  So we can't justify all the way to the 
right.  And factoring in the scrollbar size and adding some padding to 
the right isn't going to work either because if the scrollbar isn't 
displayed, the size text will be "hovering" kind of in the middle of 
nowhere and that just looks plain awkward.

The Size column in the mailbox view should be right aligned like it is 
IMP mailbox view