How to delete sender's address when onsending an email?

Not sure what you mean by "onsending", but if you mean "forwarding" a message, if you forward it as "Inline" the basic header information is included as text in the body of the message and can be edited out. If you forward a message as "Attachment" you cannot edit it.

See menu option <Message | Forward As> , or right-click on the message in the Message List and from the pop-up context message choose <Forward and Redirect | your preferred option>