Recommendations to use OVH as smtp relay with DKIM

**How to properly use OVH as relay host to send e-mails with DKIM**

Hello everyone,
My **domain name** is : waltr-air.es
And I use **the offer** : MX plan

I have **setup postfix on some servers on my internal network to use ovh smtp as relay to send mails.** This works very well. I sent can e-mails with the user name of the mail box defined on OVH.

SPF is OK but I am still missing a proper DKIM encryption.
I am asking here for the simpler and proper approche. What I have gathered so far as possible solutions:

1. Setup DKIM on my server and setup the corresponding DKIM public key on the DNS of my domain.
My worries is that I already have some DKIM keys setup there for the domain so I wonder if this is not going to be problematic. As I do not send the mail from a subdomain.
2. Variation on first approch recover the private key already defined by OVH on my domain to encrypt the mail on the server using ovh smtp as relay. I could not find in the documentation how I can recover the key so I assume it is not possible.
3. I have not found this in the documentation but make ovh encrypt the mails it received on smtp. That would be very simple but I do not think this is possible.

If you have any insight on the best way to take I would appreciate.
Regards,