The Customize New Mail Alert shouldn't be a global setting. In a workplace, if I have some personal email accounts connected to my installation, I can't afford to have personal data display while sharing the screen over the internet, presenting, or working with colleagues. There needs to be a distinct privacy option to handle these standard situations, because most of us use this app in a workplace and not just as a hobby / as a student.
A similar problem is also that when we open the Thunderbird window, if the last email account we interacted with is the personal account, those emails will be immediately in plain sight. That's zero privacy whenever we want to quickly navigate to our work emails to interact with them as part of work, as we can't simply view work emails without first showing all personal emails.
