`local_command_shell`

This parameter controls what shell will be used for delivery to external command. By default, external commands are executed directly; commands are given to `/bin/sh` only when they contain shell meta characters or shell built-in commands.

Specify a different shell when you need more control over what programs can be run from e.g. `.forward` files.

"sendmail's restricted shell" `smrsh` is what most people will use (`smrsh` is part of recent sendmail distributions)

Note: when a shell is specified, it is invoked even when the command contains no shell built-in commands or meta characters.

Example: `/some/where/smrsh -c`