That's interesting. Jetpack will indeed add a "Sent by an unverified visitor to your site." message at the bottom of the emails that were submitted by logged out visitors, but we don't remove or edit values that were submitted in the form. Akismet doesn't either; it either lets the whole message through, or doesn't send any email at all and drops the message under Feedback > Spam in your dashboard.
Could you let me know what other plugins are currently active on your site?
Could you also post your contact form shortcode here, so I can try to reproduce?
Thanks!
