Auto-forward Yahoo Mail messages to another email address
Click on "Options" (top left corner), and choose "Mail Options".
Select "POP & Forwarding" (bottom left of Options tab that opened).
Select "Forward Yahoo! Mail to another email address", and enter a full email address in the text field below it. This could be another Yahoo Mail account, or a Gmail address, Hotmail, etc.
In the dropdown menu, "Forward only" (default) "redirects" messages to the email account specified (and not keep a copy of it in your Yahoo inbox).
"Store and forward" sends a copy of all messages to your other email address, but also saves them in your Yahoo inbox (messages delivered in both inboxes).
"Store and forward and mark as read" does the same as above, but marks as "Read" any forwarded message. "Unread" then means that those emails were then not forwarded: either received before you enabled mail forwarding, or after you've turned it off (see last step).
Select another setting on the left to save your changes (or close Options tab). Yahoo confirms the new setup: "Check [email address] for a confirmation email from Member Services. Click the link in the email to enable forwarding from this account. If you don't see a confirmation email, try resending it."
Click on the confirmation link sent to your forwarding email address; Yahoo Mail will automatically add the confirmed email address to your account for password reset purposes.
To disable email forwarding, follow the same steps, but select "Access Yahoo! Mail via POP" under "POP & Forwarding". The change will only affect future emails (not retroactive).