Mail from 1 server suddenly treated as spam

We have 3 DA servers which are configured exactly the same on Centos 6.

Now since a couple of days, our main server is rejecting messages from 1 other server.
I can ofcourse whitelist this server, but I want to find the cause of the issue, because this wasn't happening before.

SMTP error from remote mail server after end of data: 550-Your message to <user@somedomain.com> was classified as SPAM. Please add 550-more content, cut down on HTML links, use fewer naughty words etc. Also, 550-ask your IT dept to make sure your mailserver has REVERSEDNS, SPF, DKIM, 550 and is not on any black lists. Your score: 90

This is even before spamd is running, because it says nothing about it in the logs.
I do see several of these in the logs:

Waing: DKIM: Invalid. reason='pubkey_unavailable'

Messages are send from admin@server.domain.tld (softaculous script update notification) to the users or from mycompany@mycompany.com which has a DKIM record on server 2.
I don't know if that's the reason of the block, because mails are send from server 1.
But if that is the case, mails are also send from that adres from server 3, so why aren't they blocked?

Neither server or domain is set into any whitelist.

Getting very confused. Mails on RBL lists is getting through and my own legal mails are blocked. LoL.