Both the A record and host -t A mail.domain.biz return the IP address. This is a local IP address because it is a local domain, and in the local DNS there is also an MX record, which is shown by the command host -t MX mail.domain.biz responding with:

Where should I look for the cause of the problem?

Hi,

Yes, you are correct that it is about redirection. In Carbonio CE, the default value for ReverseProxyMailMode is by default set to https hence the server only accepts connection through https.

zextras@mail:~$ carbonio prov gs mail.sampleserver.xyz | grep -i reverseproxymailmode
zimbraReverseProxyMailMode: https
zextras@mail:~$

Therefore, when Let's Encrypt is trying to connect to the server using HTTP protocol, and it was not been able to establish the connection. So when you try to configure Let's Encrypt please set the mode from HTTPS to REDIRECT as follows:

zextras@mail:~$ carbonio prov ms mail.sampleserver.xyz zimbraReverseProxyMailMode redirect

Restart Proxy Service.

zextras@mail:~$ zmproxyctl restart

Then, try to generate a Let's Encrypt Certificate using a Long/Short Chain.

Note: After successful Let's Encrypt Certificate deployment, you can revert the ReverseProxyMailMode to HTTPS again.

Hope it helps.

Regards,

Sharif