We are using our own smtp server.
We have solved the issue. I am not sure 100% what was the problem, but I found in another post that it was connected to proxied ips.
the post is
https://forum.mautic.org/t/click-tracking-not-working/11770
We checked, and actually we had a NAT active in our firewall, that prevented the client ip to be sent to mautic.
deleting the NAT solved immediately the issue.