Sender reputation

Every major provider keeps its own score, and it decides inbox, spam folder or rejection more than any single setting. It rises with mail people open, reply to and move out of spam. It falls with spam complaints, hard bounces, spam trap hits and authentication failures.

Domain reputation travels with you across sending services; IP reputation belongs to whoever runs the servers. You can watch parts of it in Google Postmaster Tools and Microsoft SNDS, and through blocklists.