Shell to use for delivery to external command
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