An email is visible to any participants in an email conversation (including BCC and CCC recipients), colleagues added through email comments, or those with access to a work or contact timeline the email has been added to.
The email's colleague's tab will list everyone in your team who is able to see the email.
Clicking on the briefcase icon to the right of an email will reveal the timeline tray, which details any contact or work timelines that email appears on. Whoever has access to those work or contact timelines, will be able to view that email.
If an email appears on both private and public timelines, a blue banner will appear. The private timelines will appear in the timeline tray with a padlock icon.
If you’d like to remove the email from any timelines and prevent people with access to that timeline from viewing it, you can do so from the timeline tray.