How to set up a DKIM signature for the main domain and several subdomains associated with individual IP addresses and servers?

Question

SMTP

It was required to create a tool for email marketing. We rented hosting for the site (client), 1 server for the script of e-mail newsletters + admin panel and 3 servers for SMTP. We bought a domain, tied it to a hosting and deployed a client face, created a domain and tied it to server No. 1, deployed a script on it. We created 3 more domains and each one was tied to a separate server with a separate ip address for SMTP. Servers have isp manager 5 + exim.
In the script settings, it was required to bind the domain from which the script will send letters, and after the mailbox with which the “from whom” glows, we indicated it, it gave us a DKIM key + SPF, it was registered in the settings of the main domain, checked - everything is ok. We send a test mailing from server 1 on which the script is installed, everything is fine, all mailers pass normally, we checked the letters - DKIM is specified.
We started connecting servers (also on sub-domains).
SMTP1 - sub.domain msrv1.name.ru is bound - ip 1.1.1.1 - a mailbox [email protected] is created
On the hosting, a sub.domain was also created and sent via A record to ip 1.1.1.1
msrv1.name. ru - ip 1.1.1.1
mail.msrv1.name.ru - ip 1.1.1.1
pop.msrv1.name.ru - ip 1.1.1.1
smtp.msrv1.name.ru - ip 1.1.1.1
We checked the "DKIM signature" checkbox in the mail domain, checked the records of the associated subdomain - it appeared, we start sending a test letter from this server - the mailers begin to swear, we look at the body of the letter - and we see that when sending to this server there is no DKIM signature. That is, the server does not sign letters for some reason. We are trying to versify the domain through the script, so that it shines in the "from whom" field - it gives out the DKIM + SPF key, we register it in the sub.domain record on SMTP1 msrv1.name.ru - ip 1.1.1.1, press the versify button in the script - gives an error message. After 3 hours - similarly (that is, the problem is not in updating records from providers).
With servers:
SMTP2msrv2.name.ru - ip 2.2.2.2 - mailbox [email protected] created
SMTP3msrv3.name.ru - ip 3.3.3.3 - mailbox [email protected] created
A similar story.
I can't figure out how to configure DKIM for each SMTP server, provided that the servers are on sub-domains, so that they would sign letters.
That's what the result should be, judging by this picture, each server from which the letter is sent automatically signs it.

The checker for the main domain gives a positive result, for subdomains - a negative one. I can't figure out what's wrong. Although the main script generating DKIM for subdomains - gives a positive result

Answer 1

It is necessary to sign with the DKIM of the domain from the From: field, and not with the server name. Those. you need to generate one DKIM key for each domain from From: (for example, bubuhta.ru in your screenshot), publish it with some selector, and register it and the selector on all servers that send out mailings for this domain.