Where does the incoming mail queue stores the incoming mails.

assuming, if there is an error when the mail reaches the zimbra server.

is there a way to find the error which shows if the mail is not sent through MTA.

Like a log file. zimbra.log does not record any errrors. its blank file.

Any other way to find. Can send out messages. But cannot receive it from any source, either internal to server or external mail clients.