|In Outlook, if you look under the 'Tools' menu, there's a 'mailbox cleanup' option - that lets you select any emails older than a certain date - then under the 'edit' menu you can 'select all' and delete them. They will go to your deleted items folder first, in case you want a second chance of saving any of them. If you are happy that you don't want any, then just choose to empty the Deleted Items folder.|
You can also use the 'archive' function, again under 'tools' menu, which will let you move them from the main inbox to an archive inbox, where they are still accessible, but won't slow down your main inbox. Again you can choose to archive ones older than a certain date, and also set it to be done automatically in the future.
As an additional option, under 'Tools/rules and alerts' you can set up rules for different sorts of mail to either delete, or move to another folder, or whatever - this can be run on your existing mailbox after you created the rule and do a tidy up that way.
"I've always been mad, I know I've been mad, like the most of us..." Pink Floyd