Who has the biggest influence in deciding whether your messages are delivered to recipients or placed directly in spam?

While your recipients and your ESP both play important roles, Inbox providers (also known as Mailbox Providers or ISPs, like Gmail, Outlook, and Yahoo) have the final and most significant influence over where your message lands. The Gatekeepers: Inbox providers act as the "security guards" for the recipient's inbox. They use complex proprietary algorithms and spam filters to scan every incoming email. The Decision Makers: They evaluate a massive range of signals to decide if an email is safe and wanted. If their filters give your email a "high spam score," they will automatically divert it to the spam folder or block it entirely before the recipient ever sees it.