Cannot send mail among local machines: No address associated with hostname

I am attempting to send emails between 2 machines with postfix running on both of them.

The variables myhostname is properly set to hostname-1 and hostname-2 respectively in /etc/postfix/main.cf:

$ swaks --to user2@hostname-2 --from user1@hostname-1
$ swaks --to root@hostname-2    
=== Trying hostname-2:25...
*** Error connecting to hostname-2:25:
***     IO::Socket::INET6: getaddrinfo: No address associated with hostname

What is missing here and how to send emails between local machines using their hostnames?

PS: I am able to send mail by connecting with telnet on port 25 in the 2 directions.