I may not be getting what you're trying to say.
It looks like your email software has a redirect service.
I'm using exim4 and dovecot, is that the software you are referring to?
Or are you referring to the client software?
For webmail this is likely to not leak any information through the referrer.
Are you saying that there is a problem in my browser settings?
Or that it's a general issue with browsers?
It looks like the redirection service of your webmail is broken.
Does this mean that all three of the tested webmail applications are somehow broken?
Or that the underlying email server is broken (miss configured)?
I appreciate your reply. This sort of problem is new to me and I don't have a good grasp of the interactions of the various applications. Could you give me an example of how this sort of thing might happen?