The "I Love You" email worm hit us pretty hard. We pulled the plug on the mail server (literally, pulled the power cord out of the wall!), then the network cable, brought the box back up, and spent the rest of the day cleaning up the mess, installing filters, etc.
One morning, shortly after: beautiful email from CTO, reminding users not to open email attachments, especially if they're from people they don't normally get that sort of attachment from- if in doubt, don't, etc.
Later that same afternoon:
Subject: Here you are ;-)