When you logged into your webmail, click on settings, then on Forwarder and entered your yahoo e-mail address, did it save the e-mail in that field? also are they going to both Teksavvy webmail and Yahoo or just Teksavvy?
Yes, it saved my yahoo email address in the forwarder field, but it's still delivering email to the Teksavvy mailbox as well as sending a copy to my yahoo account. If I just leave the messages sitting in my Teksavvy mailbox it will eventually run out of space. This makes forwarding email to another account pointless when I still have to download the messages to empty my Teksavvy mailbox.
Incidentally this is how some other mail programs / web mail handle forwarding these days including gmail. Not sure if there is a way to create a rule to push the mail into the trash after it is forwarded.
(Fun fact: When you set up forwarders with gmail after a few weeks and then again after a few months it sends you a little reminder to tell you that you are doing this. I think is to alert you in case someone broke into your account and setup the forwarder thinking you wouldn't notice).