[Exim] filter: how do I match locally generated mails?

Hi everybody,

I'm using exim 3.35 with the system filter.

How could I detect mails that were generated by the local host?

I tried something like

if $sender_host_name is "localhost"

which didn't work.

In the mail headers, it says:

Received from root by my.mail.server ...

So I also tried

if $sender_host_name is "root"

Surprisingly, it worked. But I'm sure, there's a better way to do this
(and also catch mails generated by other local accounts).

Thanks for your help,

Martin