When you connect or reconnect a mailbox to Instantly and it asks for a verification code, that code is sent to the individual mailbox — not to your Master Inbox (codes are not forwarded to the Master Inbox).
To read it, connect that specific mailbox to a mail client (for example in Outlook or Blue Mail, the same way you connect the Master Inbox) and open the code there.
If you're connecting many mailboxes, you can also ask Instantly support to lift the code requirement. If your account can't request a warmup/verification code at all, contact support — we can have it reactivated from the backend.
Seeing ___ ___ in the OTP field on the mailbox card in our app? That's a different thing — it just means no current code is stored for that mailbox right now, not a Master Inbox issue. Refresh the mailbox card after a moment, and if it's still blank when you need it, contact support and we can pull the current code for you from the backend.