mh-letter-toggle-header-field-display
mh-letter-toggle-header-field-display is an interactive autoloaded Lisp function in `mh-utils.el
'.
(mh-letter-toggle-header-field-display ARG)
Toggle display of header field at point.
Use this command to display truncated header fields. This command
is a toggle so entering it again will hide the field. This
command takes a prefix argument ARG: if negative then the field
is hidden, if positive then the field is displayed.