I think you're confusing the telnet program on the zenoss host with the telnetd daemon on your SMTP server.
The 25 on the end of
telnet <IP address> 25
will change the default port that the telnet program uses from 23 (telnet port) to the SMTP port
The purpose of this test is not to start keyboard interaction with the telnetd daemon (or the SMTP daemon for that matter), but simply to test if you can successfully get a TCP port 25 connection from the zenoss host to the mail server.