Of those plugins the one which *may* be causing this (if indeed it is related to a plugin) would probably be one of these:
- Contact Form 7
- Newsletter Pro
- Update Notifier
I've added some tags to this thread with the plugin names. I don't use any of these, so I'm probably not much help but perhaps you'll get lucky and one of the plugin authors or someone who uses one of the plugins has seen the issue and can advise you on a solution.
If it is happening regularly, you can test to see whether it is one of these plugins by deactiviating them in turn and seeing whether the mails stop whilst a particular one is not active.