[Rspamd-Users] Also a DKIM signing Question

[christian]

Hello,
I also have a question about DKIM.
First of all, do you know the following service: 
https://www.learndmarc.com/ - that helped me a lot (no advertising).

My question:
I have a maindomain that runs Postfix and RspamD and is registered as an 
MX server in DNS. 100 domains send their emails via this domain.

user1 at domainA.com sends via maindomain.com
user2 at domainB.com sends via maindomain.com
user3 at domainC.com sends via maindomain.com
etc.

That works, but I can't get a working Dkim signature for the emails from 
user 3 at domainC.com, i.e. all non-masterdomain email addresses.

Do I have to enter a DNS entry and a server certificate for each domain 
even though they all send via the main domain.com?
Is RspamD's dkim_signing designed for this?
I would have to use the "xxx._domainkey TEXT "v=DKIM1\; for DNS every 
time. k=rsa\; p=MIIB ....", which is an enormous job.

Thanks for Help

Christian