We are using Zimbra Collaboration Suite version 7.2.0. I have a user who has Windows XP SP3 computer with Outlook 2007 SP 3 on it. He has twice tried to send emails to someone. Both emails failed to send to the recipient and my user got the following message:

<Begin Message.
Subject: Undelivered Mail Returned to Sender

This is the mail system at host mail.cyber.com.

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 mail system

<arsalamone@mainlandfi.com>: host mx00.1and1.com[] said: 550
<arsalamone@mainlandfi.com>: quota exceeded (in reply to RCPT TO
<End Message>

When I saw the "quota exceeded (in reply to RCPT TO command)" I thought that maybe the recipient's mail box was full. I did a Google search on this exact error message. I found a page where someone got this same error message and also originally thought that this meant the recipient's mailbox was full. But upon looking into it, they said that was not the reason for the issue. Unfortunately, they took the discussion offline so I don't know how they resolved this issue.

1) Does anyone know what this error message means
2) Does anyone know how I can resolve this?

Any help would be appreciated. Thanks.