Apple email re-downloading all my emails

I have recently moved my website to a new server, and am having a lot of issues with email configuration. Among these, Apple Mail is re-downloading all the emails I have on the server (the old one I guess, which is still active). Besides, when I try to get into the advanced settings of my email account I get a pop-up showing me my hostname and asking me to type in a "path prefix" BUT I can't type anything in the space dedicated for the Path Prefix. Anyone knows what to do?

Many thanks!