mail-directory-process

mail-directory-process is a variable defined in `mailalias.el'.
Its value is

nil

• This variable may be risky if used as a file-local variable.

Documentation:
Shell command to get the list of names from a mail directory.
This value is used when the value of `mail-directory-function'
is `mail-directory-process'. The value should be a list
of the form (COMMAND ARG ...), where each of the list elements
is evaluated. COMMAND should evaluate to a string. When
`mail-directory-requery' is non-nil, during evaluation of these
elements, the variable `pattern' contains the partial input being
completed. `pattern' is nil when `mail-directory-requery' is nil.

The value might look like this:

'(remote-shell-program "HOST" "-nl" "USER" "COMMAND")

or like this:

'(remote-shell-program "HOST" "-n" "COMMAND '^" pattern "'")

You can customize this variable.