Resolved From= empty in maillog

[JuanCar]

I have a problem with my mail server and I'm trying to find the solution.
I receive failure notice from Mailer-Daemon. It looks like an unknown user in my server send an message that cannot be delivered.
I've examined the maillog and see
Feb 3 20:03:55 servername qmail-queue-handlers[4665]: Handlers Filter before-queue for qmail started ...
Feb 3 20:03:56 servername qmail-queue-handlers[4665]: from=
Feb 3 20:03:56 servername qmail-queue-handlers[4665]: to=agua-term@servername.com
Feb 3 20:03:56 servername qmail-queue-handlers[4665]: handlers_stderr: SKIP
Feb 3 20:03:56 servername qmail-queue-handlers[4665]: SKIP during call 'limit-out' handler
Feb 3 20:03:56 servername check-quota[4667]: cannot get sender domain
Feb 3 20:03:56 servername check-quota[4667]: Unable to intialize check-quota mail handler
Feb 3 20:03:56 servername qmail-queue-handlers[4665]: Error during 'check-quota' handler

I think the line from= is not normal (the user aqua-term doesn't exist)
I've checked my mail server with MxTool and there is not Open Relay

Any idea? Why from is empty?

Thanks

 

[JuanCar]

Autoreply: this is for avoid unlimited bounce.