The email seed:
Subject: [{my website} Wordpress MU] WordPress 4.2.2 is available. Please update!
Please update your site at http://{my websites url}.com to WordPress 4.2.2.
Updating is easy and only takes a few moments: http://{my websites url}.com/wp-admin/network/update-core.php
If you experience any issues or need support, the volunteers in the WordPress.org support forums may be able to help. https://wordpress.org/support/
Keeping your site updated is important for security. It also makes the internet a safer place for you and your readers.
The WordPress Team
I have the following plugins installed: All In One SEO Pack, FeedWordPress, Github Ribbon, Hello Dolly, Revision Control, Unfiltered MU, WordPress Importer and WP-Polls.
Maybe it originated from one of them?