PHP mail function sending mail to spam from OVH servers

We recently bought a Plesk reseller plan from OVH and we have created multiple web hosting plans under it. The servers are lightning fast and we are very happy with the speed of the servers. We just have a problem with forms on our websites. We build WordPress websites and the contact forms we have use PHP's mail function to send mail.

All the mail gets marked as spam though and Gmail is the only service provider who actually even receives the messages in the junk mail folder. Hotmail or our own email provider who uses a strict spam check don't even take the emails to spam folders.

We have contacted OVH support but there's no help from there. What are the DNS values we need to use for Plesk reseller web hotels? We have DNS zones from OVH manager for multiple domains where we can edit the DNS records ourselves.

Do we use mail.domain.com as MX entry or mx1.ovh.net or redirect(dot)ovh(dot)net?

Also how do we activate DKIM on the Plesk reseller plan? We have managed to put the public key to the DNS zone records and tools like mxtoolbox find the DKIM record just fine but where and how do we put the private key?

We have read a lot of Plesk documentation (which was linked to us from OVH support) and we don't have access to the settings on our Plesk interface where we could activate the DKIM on the whole server.