How to Remove Attachments from E-Mail Messages
There are many ways to decrease the size of your mailbox, and one is to remove attachments from old e-mails - in the Inbox as well as in Sent Items - without having to delete the whole message. This can be useful when you have an important e-mail that needs to be kept but you have already saved the attachment on your computer, so there is no need for the attachment to remain in your mailbox any longer.
To remove the attachment
Open the message with the attachment.
[Note: You have to double-click the mail to open it in a separate window. For some reason, Outlook doesn’t allow you to remove attachments from the preview pane.]
Right-click on the attachment.
Click on Remove.

Close the message, responding Yes to the question "Do you want to save changes?".
The attachment will be gone and your mailbox size decreased.