What is sender reputation and how do I check mine?

Sender reputation is the score mailbox providers like Gmail and Outlook assign to your domain and sending IP, based on how recipients have treated your past email - it decides whether you land in the inbox or the spam folder:

• What builds it - consistent volume, low spam complaints, few bounces, and recipients opening and replying rather than deleting on sight.
• What wrecks it - sudden volume spikes, emailing addresses that bounce, and people marking you as spam.
• How to check it - enrol your domain in Google Postmaster Tools, which shows your Gmail reputation, spam rate and authentication results for free.
• Why it is not one number - reputation is per-provider and per-IP, so a clean record at Gmail does not guarantee the same at Outlook.

Reputation is earned slowly and lost quickly - protect it by only emailing people who asked to hear from you.