Found the point of failure, although it should not happen.
According to the postfix log:
127.0.0.1[127.0.0.1]:10024: Connection refused
However, telnet tells a different tale:
root@langata:/usr/src# telnet 127.0.0.1 25
Connected to 127.0.0.1.
Escape character is '^]'.
220 mail.domain.com ESMTP Postfix (Ubuntu)