Who measures your sender reputation to ultimately decide whether or not your messages are placed into a recipient’s inbox?

Inbox providers, such as Gmail, Outlook, and Yahoo, are responsible for measuring your sender reputation. They use various factors like bounce rates, spam complaints, engagement metrics, and blocklist status to assess your trustworthiness. Based on this assessment, they decide whether to deliver your emails to the inbox, send them to spam, or block them entirely. The recipients themselves don't directly measure sender reputation; they only react to the emails they receive, which then influences the inbox providers' algorithms. Klaviyo, as an email service provider, helps you manage your sending practices and provides tools to improve deliverability, but it does not ultimately decide where your messages land.