Weird emails returned

Suddenly I'm getting old emails returned, messages that I wrote, or may have wrote some weeks/months ago. The return is from postoffice at address.invalid and the delivery address is armisis at dsl.pipex.com. I have absolutely no idea who or what that is. Has anyone else seen this kind of stuff or have any idea what is going on? My guess is someone using Outlook may have some virus/trojan playing games with their email. Just Curious, Richard -- Old age ain't for Sissies!