It's a long time since I suffered Outlook so my experience may not only be imperfectly remembered but thing may have changed. However AFAICR it shares a basic problem with the email client I've used at home for years: an ill-thought out UI. Ill thought-out, that is, in terms of what the user might reasonably want to do with an incoming email.
In the physical world reading a message from an in-tray involves physically removing it. Having read it the recipient can do one or more of several things: reply, forward it, bin it or in extreme cases shred it, put it to one side to be dealt with later* and file it.
With email there are options to reply, forward a copy and delete. Reading doesn't even remove it from the in-tray.
The only readily available options for disposing of the item are either to leave it in the in-try resulting in a bulging in-tray (guilty as charged m'lud) or delete it, the latter leading to stories like this. Neither is appropriate.
A start at a better UI would be to remove the item from the in-tray. The user can dispose of it by moving it to a delete bin (to be emptied later according to time or volume rules), shredded (deleted immediately and irrevocably) or moving to a pending tray. If the user does nothing it will go into a filing folder.
Being computerised there are filing options that can be automated. Threads of liked messages can have their own folder. Emails to and from specified domains can have a folder specified; this would be useful for clients, suppliers, the bank etc. Specific addresses or groups of addresses in the address book can have a folder specified. Ad hoc folders can be specified so there can be a hierarchy with, for instance, individual thread folders grouped in an ad hoc project folder or specific client folders grouped in an overall Clients folder. Sent mails, including those which forward a message, would also go into the filing area. There could, maybe, be two holding folders, read and sent for anything not yet dealt with under rules.
To some extent I can do this with filters on the Thunderbird/Seamonkey client but there are a few of annoyances, the first being that it isn't the default action for disposing of a read email.
* It could be left in the in-tray or added to a pending tray.