Go to Settings -> SMTP and Mailgun Service
Limitation if you are using SMTP provider like Outlook:
When a lead reply to the emails sending from HighLevel, it comes back to the conversation tab, the system then attempts to forward the response email from the lead’s email address (masking)
We are essentially using your SMTP using the lead’s email as the sender email. So it gets flagged because the from email is the lead’s email and not yours during the forwarding process.