My e-mail is sending mail on it's own

The last week I’ve been getting strange e-mails saying that my messages sent to so-and-so were undeliverable. The thing is, I’m not sending anyone any e-mail. Is someone using my account to send out messages withou t my knowlege? Here is an example of the response I’m getting

**- These recipients of your message have been processed by the mail server:; Failed; 5.1.1 (bad destination mailbox address)

Remote MTA SMTP diagnostic: 550 5.1.1 <> is not a valid mailbox; Failed; 5.1.1 (bad destination mailbox address)

Remote MTA SMTP diagnostic: 550 5.1.1 <> is not a valid mailbox**

Has someone gotten into my e-mail? I have run virus scans to no avail. I am about to change the password on my account. Can someone help figure out what’s going on? The message I am sending is this:
**From: Florence Blake []
Sent: Saturday, April 03, 2004 1:19 PM
To: Wescott
Subject: Fwd: Prompt. No Embarrassments. Val-i-um.V~|cod|n.v!@GRa._X_ANAx.NQUXADOJ

He may sell field, garden, and house to a merchant (royal agents) or to any other public official, the buyer holding field, house, and garden for its usufruct.

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No longer yours, than you your self here live,
For that deep wound it gives my friend and me;
To stand in thy affairs, fall by thy side.
Ah, if thou issueless shalt hap to die,**

I have the same problem

However it went away for some reason, and only happened for one day.

Guys… those aren’t real. Your email server didn’t send any messages. Hell, chances are, if I’m right, those “Undeliverable” messages said that the email is in the attachment, or had one. Those are virii, not real undeliverable messages.

Xash explains it all.

The plural of virus is not virii, no matter how you look at it.