IP reputation is the trust score that Internet Service Providers assign to a sending IP address based on the historical behaviour of emails sent from it. It is one of the two primary signals — alongside domain reputation — that ISPs use to decide whether your email reaches the inbox, the spam folder, or is blocked entirely.
How IP Reputation Works
Every email sent from MigoSMTP travels from a specific IP address to the recipient's mail server. That mail server — whether it's Gmail, Outlook, Yahoo, or a corporate mail gateway — checks the sending IP against its own reputation database and multiple third-party reputation services before deciding what to do with the message:
- Your application sends an email via MigoSMTP SMTP or API.
- MigoSMTP dispatches the email from one of its sending IP addresses.
- The recipient's mail server receives the connection and looks up the sending IP.
- It queries its internal reputation data and external reputation feeds (Spamhaus, Barracuda, Talos, etc.).
- Based on the IP's history, the server makes one of four decisions.
| Decision | What Happens | Reputation Signal |
|---|---|---|
| Accept and deliver to inbox | Email delivered directly to the primary inbox | High reputation — trusted sender |
| Accept and route to promotions/spam | Email delivered but placed in a filtered folder | Medium/Low reputation — uncertain or marginal sender |
| Defer (4xx response) | Email temporarily rejected — MigoSMTP retries automatically | Low reputation — throttling the unknown sender |
| Reject (5xx response) | Email permanently rejected — bounces and is not retried | Bad reputation — IP blacklisted |
IP Reputation vs Domain Reputation
| IP Reputation | Domain Reputation | |
|---|---|---|
| Tied to | Sending IP address (e.g. 203.0.113.42) | Your sending domain (e.g. mail.yourcompany.com) |
| If you change SMTP providers | IP reputation resets — new provider's IPs need warming | Domain reputation travels with you — it is yours |
| On shared IP (MigoSMTP default) | Shared with other senders on the pool — partially affected by their behaviour | Entirely your own — 100% under your control |
| Recovery speed | Faster — can move to a new IP and start fresh in weeks | Slower — blacklisted domains take months to recover |
| Modern weighting by ISPs | Important but secondary for established senders | Primary signal for Gmail and Yahoo since 2024 |
How ISPs Score IP Reputation
No ISP publishes its exact reputation algorithm, but the factors with consistently demonstrated influence are:
- Volume history — a steady, consistent sending pattern over time builds trust. New IPs and dormant IPs are treated with initial suspicion.
- Complaint rate — the proportion of delivered emails that recipients mark as spam. This is the most damaging single metric.
- Bounce rate — particularly hard bounces to invalid addresses, which signal poor list quality.
- Spam trap hits — sending to spam trap addresses (maintained by ISPs and anti-spam organisations to identify senders with dirty lists) causes immediate reputation damage.
- Engagement signals — for Gmail especially: emails that are opened, replied to, or moved to inbox from spam contribute positive reputation signals.
- Authentication — IPs that consistently send authenticated email (SPF + DKIM + DMARC passing) receive more trust than those sending unauthenticated.
Reputation Scores — What the Numbers Mean
Third-party reputation providers use numerical scores. The most widely referenced is the Sender Score from Validity (formerly Return Path):
| Score Range | Interpretation | Typical Inbox Impact |
|---|---|---|
| 91–100 | Excellent — highly trusted IP | Strong inbox placement with minimal filtering |
| 71–90 | Good — generally trusted | Good inbox placement; minor filtering possible at some providers |
| 51–70 | Fair — marginal trust | Inconsistent delivery; some messages to spam folder |
| Below 50 | Poor — flagged as problematic | Most messages to spam; blocks and rejections common |