I answer a lot of the same questions a week….
Unfortunately it appears that this is backscatter from a recent spam mail out. Just to be clear it does not appear that your systems were involved in sending the spam. What has happened is that a Spammer using their own system has sent spam to a 3rd party. The spammer forged the “from” address so that any errors generated, for example a non-existent email, come to you. The address chosen by the spammer is normally completely random.
As none of the activity takes place on your system we are extremely limited in the mechanisms that we can implement to automate removing this problem. Most spammers change “from” addresses frequently and as such the incoming messages should stop reasonably quickly. Blocking all such messages would prevent notification of real failures from reaching you. If the problem re-occurs frequently for the same user/email address please let us know so we can look into some more extreme measures.
For more information please see http://en.wikipedia.org/wiki/Backscatter_(e-mail)
Spam Email Levels:
Unfortunately we have seen a rise in the levels of spam at the moment, we are as always looking for ways to reduce the number of spam emails that reach your inbox however there is no perfect system. We will look at adjusting the filters to catch more however we must always balance the level so as not to catch legitimate mails.
Thanks for letting us know however as it is important that we know it is affecting you.