@GO3 -- what E-mail program are you using?
Within Shaw WebMail, if you have received a message with a large attachment, open the message, download the attachment to your "C:" disk-drive, and then click to "remove" the attachment. Close the E-mail message, and the size of the message will be greatly reduced.
If you are using Microsoft Outlook, there is the option to create "Personal Folders" on your "C:" disk-drive. Then, drag-and-drop messages from your INBOX into those folders, to reduce the amount of space being consumed by your Shaw E-mail messages.