Is this a virus?  I keep getting messages returned from Mailer Daemon
and Yahoo, maybe one other, and they're nmessages I never sent to at
addresses I've never heard of.  Every day, several arrive.  What's the
cure?

Hello,

Are there any attachments being sent with these returned messages? If
there are, be sure not to open them, it could be that a virus is being
sent to you.

It could also be the case that you do have a virus and it is sending
out messages to other people, but this is not as likely, considering
viruses usually send out messages to people in your mailing list.

If you want to check your computer and remove any viruses, try this
free online virus scan:

Trend-Micro HouseCall
http://housecall.trendmicro.com/housecall/start_corp.asp

You might want to make a folder in your webmail or email application
and create a rule that makes any message containing "Mailer Daemon" go
automatically into that folder. You can then check the contents once a
week or so, and dump the contents. This will save you time sifting
through your email.

If this isn't the answer you were looking for, please let me know and
I will further address your question.

Regards

tisme-ga

Search Strategy:
Personal/Knowledge Experience

I have had this happen to me. There is a particular kind of virus out
there these days (klez and similar ones) that will send viruses to all
the people that it can, and fake the return address. What this means
is that say someone has my email address and is infected with this
virus. They will send out this virus to everyone on their mailing
list, and fake the return address as mine. Then, when other people get
the virus, a message is sent to me saying that "my" email was rejected
due to a virus. It is possible that your computer is infected with
this virus, so you should update your antivirus to make sure, but it
is very possible you have nothing to worry about.

It is more likely that your address was pirated by a spammer, 
 who is sending thousands of messages with faked  sender address (yours)
 to a list.
 When an address on the list is 'bad' you get a mailer demon notice.
 When it is 'good' - people just think that you are the spammer.. :-(

 Practice is illegal but widespread.

 For help, visit anti-spam sites

spam.abuse.net/

  To identify a virus, visit 

http://hoaxbusters.ciac.org/HBHoaxInfo.html
http://www.internet-101.com/hoax/

It is impossible to be fully sure what the problem is without further
information, but I believe that out of the 3 possibilites outlined in
both the answer and the comments, at least one is responsible for your
problems.