no route to the host - Postfix - problem occurs only with IPs in the same network

Hi, I am a newby in the topic and I would appreciate any help…

I have just set up two VM with Virtualmin gpl on the server. First one works as the webserver and the second one as the mailserver for the same domain. They have two different IP’s in the same network (x.x.x.39 and x.x.x.42). Everything works fine apart of the automated mails sent from the website set on the webserver (x.x.x.39) to the mailboxes on the mailserver (x.x.x.42). The mail stuck in the Postifx queue with the error: “no route to the host”.

The mails send by webserver (x.x.x.39) to outside the network work fine.

There are also no other issues at all with the mailserver (x.x.x.42), it receives and send internal and external mail with no problem.

I tried to ping the (x.x.x.42) from the (x.x.x.39) and it is unreachable.
From outside there is no issues with pinging it or telnet on the port 25.

Any suggestions?
Please help,