[exim] DKIM outgoing Mail

Hello,
I do sign outgoing mail with dkim.
In my config I use

remote_smtp:
driver = smtp
dkim_domain = ${lc:${domain:$h_from:}}
dkim_selector = mx
dkim_private_key =
/etc/exim4/ssl/dkim.${lc:${domain:$h_from:}}.private.key
dkim_canon = relaxed

all is fine for my domains I do hosting.

but when i get a mail from facebook and do a redirect to an other
mailadress I get an error that the private key for facebook does not exist.

2017-03-01 04:01:22 1ciuVy-0004Hv-Ad <=
notification+kr4ynkwxkesx@??? H=(mx-out.facebook.com)
[69.171.232.172] P=esmtps X=TLS1.0:ECDHE_RSA_AES_128_CBC_SHA1:128 CV=no
S=20050 id=a7d60ae185f5cf2f9aa2e4db61bfeac8@???
....
2017-03-01 04:01:24 1ciuVy-0004Hv-Ad unable to open private key file for
reading: /etc/exim4/ssl/dkim.facebookmail.com.private.key


That's ok. but how do i solve it.
Should i do only sign my own domains?

best regards,
basti