you can also click on options on the email to switch outgoing account, as well as do other annoying things like request read receipts, set importance etc.
the only way i know of to seperate accounts is with filters
Tools > Rules Wiz
or
Tools > Organize
also, here is an awesome add in for backing up your outlook file (best place is to a different computer over network share)
http://office.microsoft.com/download.../pfbackup.aspx
anybody know of any other cool add-ins?