It sounds like these are emails coming from your specific inboxes vs a marketing email, correct? If that is the case, those emails are being sent by your mail server, not HubSpot’s servers. If it is happening consistently with the same recipients, please check with them to be sure you are allowlisted. If it is consistent when you send to several organizations, I would recommend checking with your email service provider.
I know it doesn’t solve the immediate solution, but I hope it at least points you in the right direction.