If you are using Mac Mail (as opposed to WebMail Through a Server) that message should read, " I could not make a connection with the Server".
Since Mail only knows how to remedy that by asking for a new password, that is what it does.
Other issues include that the sending or receiving server is currently unavailable, perhaps it has crashed, been take offline for backups or maintenance, or a software problem at the ISP end makes it temporarily unusable.
Before you get all worked up, about it, just try again later.