Sender reputation is the trust score that mailbox providers (Gmail, Outlook, and others) assign to your sending email address and domain. It's based on your sending history: how often your mail bounces, how many recipients mark it as spam, how consistent your volume is, and whether your domain is properly authenticated.
A strong sender reputation means your emails reliably reach the inbox. A damaged one means more of your mail gets routed to the spam filter or rejected outright — hurting your email deliverability even for legitimate, well-written messages to valid recipients.
You protect your sender reputation by keeping your bounce rate low (verify addresses before sending), avoiding sudden high-volume blasts, writing genuine non-spammy messages, and setting up proper authentication. For a job seeker, the practical rule is: never mass-blast guessed addresses, because one bad batch can quietly poison every future email you send.