DKIM Header Not Added to Outgoing Emails – Plesk Server

Question

I am using a Plesk server to host my websites and also serve as the mail server for these domains. I have activated DKIM on the server and added the appropriate DNS entry for the DKIM selector. Despite this configuration, the DKIM signature header is not being added to outgoing emails. This leads to the emails failing the DKIM test and sometimes landing in the spam folder.

Here are the steps I have taken so far:

Activation of DKIM in the Plesk email settings. Adding the DKIM DNS entry as suggested by Plesk. Verification of the DNS entry for correctness. Despite these measures, the DKIM header is not attached.

I tested the outgoing mail with mail-tester.com and received this result:

"DomainKeys Identified Mail (DKIM) is a method for associating a domain name to an email message, thereby allowing a person, role, or organization to claim some responsibility for the message."