DKIM: how to configure ispconfig for send email for a external email

From my ispconfig server I must send many emails for a external domain (es: sub.domain.tld), managed from another email server and not configured into ispconfig (MX point to another email server).
How to I can configure ispconfig to send email containing the DKIM key for this domain?
It's possible configure it without install and configure the opendkim package and modify the postfix config file?
Many thanks
Dario

 

Copy the DKIM private key from that other server. But better may be to make you server send via that other server, that is use it as a smart host. https://en.wikipedia.org/wiki/Smart_host

 

Thank Taleman,
I think I could create in my ispc a new domain with same name like external domain, then generate and enable DKIM key.
Send the DKIM public key to external domain customer, telling them to insert it in their DNS, together with the SPF record containing the IP of my server.
In this way when my ispc server send email with [email protected], outgoing emails will marked with DKIM.
what do you think of this approach
Many Thanks.
Dario

 

DKIM public part of the key is indeed published in DNS. The private part of the key must recide on the server doing the signing.

In what way is that different from the ordinary setup? In your first post you wrote

If that is not the case, the setup is easy. If you have two e-mail servers for the same e-mail domain, you have to do extra setup. Are the e-mail servers both maintained by you? Why two?