Sender reputation

Sender reputation is the running judgement each mailbox provider keeps on your sending domain and IP. There is no single global number: Gmail, Outlook, and Yahoo each score you privately, and Google Postmaster Tools exposes its version as Bad, Low, Medium, or High. The recipe is not published. What providers describe in general terms is a mix of domain and IP history, spam complaints, hard bounces, spam trap hits, authentication results, how steady your volume looks, and how recipients behave (replying, clicking, dragging you out of spam). How those combine, and which one weighs most on any given day, is provider-specific and deliberately undisclosed.

Watch it whenever mail matters to the product, and watch it per stream. Rough thresholds to steer by: complaints under 0.1% (0.3% is the hard ceiling Gmail publishes), hard bounces under 2%, unsubscribes that stay flat. Reputation attaches to the domain as well as the IP, which is why isolating streams on subdomains keeps a bad marketing week from sinking password resets.

The misconception worth killing: reputation does not reset when you switch sending providers. The IP changes; your domain, your list, and your content do not, and domain history follows you across the move. Whether a particular provider leans harder on domain or on IP history is not something you can read off a dial, so assume both matter. It also decays asymmetrically, falling in a single unthrottled blast to a purchased list and taking weeks of clean, engaged sending to climb back.