Variable: imap-default-user
imap-default-user is a customizable variable defined in imap.el.gz.
Value
"root"
Documentation
Default username to use.
Source Code
;; Defined in /usr/src/emacs/lisp/net/imap.el.gz
(defcustom imap-default-user (user-login-name)
"Default username to use."
:type 'string)