Auto BCC copies addresses you choose into the Bcc field of new emails from an account. Via Newton Mail adds a short "Sent via Newton Mail" line to the emails you send from an account. Both are set for each account, and both are off until you turn them on.
Steps
Desktop
Open Settings: press ⌘, on Mac, or Ctrl+, on Windows and Linux.
Go to Accounts and select the account.
Under Email signature, switch Via Newton Mail on or off.
Under Auto BCC, type an address in Add an email address and click Add. Click × next to an address to remove it.
iPhone, iPad & Android
Open the menu and tap the Settings gear at the bottom.
Tap Accounts, then the account.
Switch Via Newton Mail on or off.
Tap Auto Bcc, type the addresses separated by commas, then tap Save Auto Bcc.
Select the account and switch Via Newton Mail on or off.
Click Manage Auto BCC, type the addresses (separate several with commas) and click Add.
Good to know
Auto BCC fills in the Bcc field when you start a new email from that account. You can remove the addresses before you send. It isn't added to replies or forwards.
The "Sent via Newton Mail" line is added when the email is sent, so you won't see it while you write. It goes under your message, above any quoted earlier messages.
Both settings apply on all your devices. On a shared team mailbox, a team admin sets them.