If you have apparmor or the firewall enabled it could be causing you problems, it's not recommended to run either on the Zimbra server. The error you've posted is indicative of a problem at the receiving server, howevere I can connect to those servers without problems. You can check the connection with telnet by FQDN or IP like this:
Code:
telnet 65.55.92.184 25
telnet mx1.hotmail.com 25