Firewalls may block marketing emails for several reasons:
IP Blacklisting: If your sending IP is blacklisted, firewalls will block emails originating from it. Content Filters: Emails containing certain keywords or phrases can trigger content filters, causing the email to be blocked. High Bounce Rates: A high number of bounced emails can indicate to firewalls that your emails are spam. Sender Reputation: Poor sender reputation can result in your emails being blocked by firewalls.