[anand]

if you see a SEND_FAILURE, specifically with this text "not sent to any recipients because these failed addresses", it is time to check /opt/zimbra/log/zimbra.log (or /var/log/zimbra.log, depending on the release). The webapp (inside tomcat) contacted postfix (via JavaMail APIs), and issued a RCPT TO: command, and postfix reject the RCPT TO:. The reason why postfix rejected the RCPT TO: should be in the logs along with the exception stack trace for the SEND_FAILURE (follow the causing exception chain).

Usually when this happens, specially with a @gmail.com address (ie, a common and well known domain), I begin to wonder if you have reject_unknown_hostname (in admin console Global Settings/MTA tab) enabled and DNS disabled. If that is true postfix not be able to DNS lookup gmail.com and will reject the RCPT TO: command. I am not saying this is your problem - consider this an example of how this failure might occur.