Any DKIM Tuto rial

Hey,

I’m trying to add a DKIM signature in my mail but I don’t realy know how to do this.
I did add and set up DKIM on my debian server and in my domain DNS but mail-tester tell me the DKIM header is not added in the mail.

I would appreciate if anyone can help or have a tutorial for DKIM with contact form 7.

Cheers