公司server上網

I am assuming your question is asking "if you use Yahoo Mail at work, will your company be able to see the content?"

Well, this is a difficult one. Unlike your company email that goes through their email server, your Yahoo Mail is like any other web pages. So what that mean is, it is possible for them to have a record of your browsing history including the use of Yahoo Mail.

It is down to how your company's internet connection is setup. If they use a Proxy server then the chance is all your internet activities are being monitored, to what level is down to how paranoid they are. Even if it is not obvious there is a proxy server, don't assume the connection is not being monitored.

To be perfectly honest, it is unlikely anyone would store this type of information. Most Proxy servers are there to filter danger sites rather than to store individual's activities.

So it is possible, however I believe companies are supposed to make users aware of any type on monitoring (at least that is the standard practice in the UK).