When I try to send an e-mail to the one specific domain I'm getting an error message:

Primary target IP address resonded with: 554 policy violation. attempted failover to alternate host, but that did not succeed etc..

I have no problems with sending e-mails to most of the domains, exepct this one

What might that be?

Thank you in advance

Hi

What settings do you have on your send connector?

Hi 

I think this seems to be a problem at the receiving end 

Things that might be wrong on your end as well which you need to check below 

Do a (rDNS, telnet to your server to check the FQDN and IP Blacklists check)
Try creating a dedicated send connector for this domain alone for this address space alone and see the results . Select the option offer basic authentication only after starting TLS and see the results 
Check in your fire wall if this type of connection "Private command : X-ANONYMOUSTLS" SMTP Policy Restrictions is set . You should make the modifications to the firewall to allow this traffic.
In your case i think mostly it should be the fire wall not allowing X-AnonymousTLS type connections 

Also 

The value  that is set on the Send Connector. It should match the external host name that resolves to your external IP address AND have set as the PTR on the IP address.