[exim] DKMI signing in EXIM

Hi..

I have exim mail server that works with mailman ,

im trying to implement DKMI signing for outgoing mails from this server ,
i only want to implement DKMI signing only to one domain say "example.com"
, not for the all the other domains

In split configuration mode , in config file
/etc/exim4/conf.d/transport/30_exim4-config_remote_smtp ( OS : Debian
exim version 4.72)

i added the following lines to end of the file ,

dkim_domain = example.com
dkim_selector = email
dkim_private_key = /etc/exim4/dkim.private.key
dkim_canon = relaxed

but when i tested , mail servers signing all the messages that going via it


Is there any config changes that i need to make this work ?

Thank You