Yahoo Mail (mail.yahoo.com) allows you to retrieve mail from other (non-Yahoo) mail servers using the POP3 (Post Office Protocol version 3) standard. However, Yahoo Mail uses an version of POP3 which is ancient and doesn't work well with modern mail servers, like the one use by WCNet.
The current POP standard is defined by the RFC (Request for Comment) document #1939, also known as Internet Standard 53 (or STD-53). That specification is from May of 1996.
Yahoo Mail servers attempt to use the POP3 LAST command when retrieving mail from a POP3 server. However, that command was removed from the POP3 specification in RFC #1725, in November of 1994.
The LAST command was removed for a number of very good reasons. See this page for additional details.
Yahoo has had over six years to solve the problem but hasn't bothered to do so.
What this means is that when you fetch your WCNet mail via Yahoo Mail, you must fetch ALL of your mail each time. So you'll want to make sure that Yahoo Mail is configured to delete mail from WCNet's servers after it is retrieved.
There are a number of things you can do if this is not acceptable to you:
Use a competing service
By using Microsoft's HotMail you hit Yahoo where it hurts AND get to use a service that has kept up to date with changes in the POP3 protocol.
Complain to Yahoo
Yahoo does listen to complaints. And if enough people complain, they may spend the small amount of effort required to fix the problem.
Use WCNet's MailMan
MailMan (mailman.wcnet.org) is a Web-based mail reader that you can use when you're away from your normal computer.
Yes. I've spoken with the lead producer for Yahoo Mail as well as one of their long-time engineers. The acknowledge the problem but have no plans to fix it. The way they see it, if you're going to use Yahoo Mail, they'd like you to use it exclusively.