Right-click the mail link (which should read something like "mailto:someone@somewhere.com") and select "Copy Link", then paste it into the "to" field in your webmail client. Then, remove the "mailto:" from the beginning of the address.
Safari will not open your webmail client by default -- all "mailto:" links automatically open the operating system's default mail client application. A webmail client is not a "mail client application" so it won't open by default.
I want to say that I remember running across a Firefox plugin at one point that modified the behavior of this and opened your webmail client page instead, but I can't seem to find it now. Perhaps it was simply a fig newton of my imagination.