If an email or note is on the timeline of public contact and/or their work, all your colleagues will be able to see it.

When you are in Triage, you’ll know an email is on a timeline and public to everyone in your team by looking at the last tab of the information tray.

If you'd like to keep the conversation private, just deselect the timelines by clicking on the "x".

Further reading:
Overview of Privacy

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