Sender reputation
Sender reputation is the trust score a mailbox provider gives your domain and IP address, based on how recipients and servers react to your mail.
Mailbox providers keep a record of every domain and IP that sends them mail. The record is built from signals: how many messages bounce, how many people mark them as spam, whether anyone replies or opens them, whether you hit spam traps, and whether your authentication is correct. Good history lets mail through. Bad history sends it to spam or blocks it.
Today the domain matters as much as the IP, and often more. If you send from Google Workspace or Microsoft 365, you share IP ranges with others, so your own domain carries most of your reputation. This is why one bad campaign can damage a domain for weeks.
You cannot see one single score. Use Google Postmaster Tools for Gmail, and watch your bounce rate, complaint rate and replies. To protect your reputation: verify lists, keep volume steady, make it easy to opt out, stop sending when numbers turn bad, and keep cold email off your main business domain.
In Terlido
Terlido checks each Outreach mailbox's warmup spam placement every hour. A mailbox that lands in spam is marked as resting, and Outreach stops sending campaign mail from it while warmup continues.