I'm part of a few organisations on GitHub and had one email address for notifications. This annoyed me as I didn't want company notifications to my personal account as I couldn't answer these. As it turns out, you can set up several email addresses in your GitHub account. In settings, go to Access > Emails and you can add more than one address.

If you then go to the Notifications settings of GitHub and scroll all the way down to Custom Routing, you can define which organisation should send notifications to which email account.

Thanks to my colleague Anton Ross for showing me this!