If you get hundreds of emails every day your inbox can fill up quickly.
To get to this window, punch Tools \ Mailbox Cleanup.
In this window you have several options to choose from.
The easiest option is to select AutoArchive, which will move older items into the Archive Folder.
You could also choose to empty the deleted items.
Deleting all the largest emails would very quickly bring your inbox size down to a reasonable level.
