Why am I receiving messages that read,
"A problem was found in an Email message you sent." when I did not send an email to that person?

Email Message:
A problem was found in an Email message you sent.
This Email scanner intercepted it and stopped the entire message reaching its destination.

The problem was reported to be:
Illegal MIME chars found in header
Please contact your I.T support personnel with any queries regarding this policy.

Answer:
Someone that has you in their address book has a virus. The virus randomly picks people from the address book and uses the addresses in the FROM and TO fields. If the virus picks a bad address for the TO address, it gets sent back to the FROM address. In this case, you were picked for the FROM address. It is fairly common, but only means that someone that you probably know has a virus.