I am using zimbra 8.0.4, it is a new installation.
Everything works fine except some emails are always in deferred status (not all). I have tried to requeued them but after several seconds they are placed back to deferred queue. From /var/log/zimbra.log, I found following warning messages:
warning: email.XXXXXXXXXX.com [xxx.xxx.xxx.xxx]:25 offered null AUTH mechanism list
warning: hash:/opt/zimbra/conf/relay_password is unavailable. open database /opt/zimbra/conf/relay_password.db: No such file or directory
warning: hash:/opt/zimbra/conf/relay_password lookup error for "XXXXXXXXXX.com"
warning: 856A76DA4413: smtp_sasl_passwd lookup error