If you receive a notice from the Systems administrator indicating your mailbox is over its size limit, or if you simply want to do some routine maintenance on your mailbox, follow these steps to decide where to focus your attention.
Note: It is almost impossible to exceed the size limit of a mailbox with simply e-mail messages. However, it is very easy to fill it up with inserted files. As you go through these steps, look for inserted files. Delete the ones you don't need anymore, and save the ones you do onto your hard drive or network share.
If you don't currently have the folder list visible, open the View menu and click Folder List.
Right-click on the top level folder (the one that reads "Mailbox - Your Name"). From the shortcut menu that appears, select Properties for "Mailbox - Your Name".
Click the button on the lower left labeled "Folder Size..."
This dialog box will show the overall size of the mailbox, as well as the size of all the subfolders. The symbol Size(Fld/Fld+Sub) stands for "Size of the folder / size of the folder plus its subfolders". If a folder has no subfolders, the same number will appear twice.
Scroll through this list to see which of the folders are the fullest. Note these so that you can examine them in more detail next. In the example above, the Sent Items folder, with its 9MB, accounts for over 1/3 of the 24.6MB of the whole mailbox.
Close this box by clicking Close. Click OK at the Properties dialog box.
Return to the Folder List and open those folders which showed to be the largest from the step above.
You can sort the contents of the folder by clicking on the column headers above the information viewer. Click on the column header for attachments (the paper clip icon) to sort the items by whether or not they have an attachment. Review the messages that do have attachments and either delete the message entirely or save the inserted files on your hard drive or to a network share, and then delete the message.
To save the inserted file, open the message by double clicking it. The icon for the inserted file will appear as part of the message. You can double click on it to open it up in the associated application, or right click on it and select Save As... from the shortcut menu.
Once you've saved the file, delete the message.
An even more direct way to find the largest messages is to add the Size field to the view. You can add fields to the view by right-clicking anywhere on the column header bar and selecting Field Chooser from the shortcut menu.
Scroll down until you see the Size field. Click it and, while holding down the mouse, drag it up to the column header bar.
Then click on the size field to sort the contents of the folder by size. By deleting the largest items, you can quickly clean up any folder.
Be sure to empty the deleted items folder (Tools/Empty Deleted Items). To see how much space you cleared up, repeat step one.