I am new in UNIX system. I added new e-mail in e-mail list and built new aliases database using newaliases command. List works fine but it kicks back error message every time someone sends e-mail. I attached the error message that sender got.
-----Original Message-----
From: Mail Delivery System [mailto:MAILER-DAEMON@hostname]
Sent: date
To: sender
Subject: Undelivered Mail Returned to Sender

This is the Postfix program at host hostname.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to <postmaster>

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The Postfix program

<#@hostname> (expanded from <email@hostname>):
unknown
user: "#"

<Name@hostname> (expanded from <email@hostname>):
unknown user: "Name"

<Othername@hotstname> (expanded from <email@hostname>):
unknown user: "Othername"