Yahoo d e-mail過去左outlook,就睇唔番?????

No method. as Yahoo! doesn't provide IMAP services, you cannot leave the message on YAHOO! account, once you download the email, the email in yahoo will be deleted.

There is another will help.
[TOOLS] > [ACCOUNT] > double click on YAHOO account > [Advanced] > Leave the message on Server.

Now you can set the date-length how long the email going to keep.