Undeliverable email

a translation:
they tried to use your email for spamming / phishing and it didn't work.

or, it's a phishing attempt to find out what they can from you while you try to find the message origin.

reading the full header info will help you to find the true origin, and also maybe looking at the attachment that is the failed message.
be carefull with last it could infect with virus / rootkit.

General protocol error (for example, the remote SMTP server responded to EHLO with a 500 level error, and the sending system QUIT the connection. The server reported this with an NDR indicating the remote SMTP server can?t handle the protocol).

resolution

Perform an SMTP log or Netmon trace to determine why the remote SMTP server rejects the protocol requests.

Non-delivery reports (NDRs) are system messages that gets generated whenever a message cannot be delivered. If the computer can detect the reason for the failed delivery, it maps the reason onto a status code, and a corresponding error message is printed.

For NDRs, most numeric codes are reported in the form of 5.X.X (Similar to your case) and are described as permanent failures.

Now, according to this page:
http://support.microsoft.com/?kbid=284204

The numeric code: 5.5.0 means Generic protocol error (SMTP error)

Possible Cause: The remote SMTP response to our EHLO with a 500 level error and the sending system will QUIT the connection and report this with NDR indicating the remote SMTP server cannot handle the protocol. (For example, if a Hotmail account is no longer active, a 550 SMTP error will occur.)

To Troubleshoot: Run an SMTP Log or a Network Monitor trace to see why the remote SMTP server rejects the protocol request.

I hope it is clear for you now.

TK