Sender reputation is the score mailbox providers keep about you. It is not one number you can look up, and there is no single authority that sets it. Each provider keeps its own view, built from how your mail behaves. If it is good, your messages are trusted. If it is poor, even a perfectly written message gets filtered.
Two kinds of reputation
IP reputation attaches to the server address your mail comes from. It was the main signal for many years. If the address sends spam, it is penalised, whoever owns the domain in the From line.
Domain reputation attaches to the domain in your DKIM signature and From address. It follows you if you change servers or providers, which is why large providers now weigh it heavily.
Both matter. Domain reputation is the more durable of the two, because moving to a new IP address does not clear your history.
| IP reputation | Domain reputation | |
|---|---|---|
| Attached to | Sending server address | Signing and From domain |
| Shared with others? | Yes, on shared IPs | No |
| Survives a provider change? | No | Yes |
| Built by | Volume and behaviour from the IP | Authenticated mail from the domain over time |
This is why authentication matters. A domain can only build its own reputation if receivers can reliably tell the mail is yours.
What providers measure
Providers do not publish their formulas, but the inputs are well understood:
- Spam complaints. Recipients clicking "Report spam" is the heaviest negative signal.
- Bounces. A high rate of invalid recipient addresses suggests a careless or purchased list.
- Spam traps. Addresses that exist only to catch senders who collect addresses without permission. Hitting one is serious.
- Engagement. Opens, replies, moving to a folder, and starring are positive; deleting unread is negative.
- Volume patterns. Steady is good. Sudden spikes look suspicious.
- Authentication. Consistent pass for SPF, DKIM and DMARC.
- Content and links. Domains and URLs in your messages are judged too, and linking to bad domains counts against you.
Shared versus dedicated IPs
On a shared IP, you borrow the reputation of every other sender on it. A well-run provider keeps it healthy by removing abusers. On a dedicated IP, the reputation is entirely yours, which helps if you send high volume and hurts if you send little, because there is not enough traffic to build a record. For low or moderate volume a well-managed shared infrastructure with your own authenticated domain is usually the better choice.
How to check your reputation
- Google Postmaster Tools shows domain reputation, spam rate and authentication for Gmail recipients.
- Microsoft SNDS and JMRP give data for Outlook.com and Hotmail.
- Public blocklists such as Spamhaus and others list addresses and domains. Look up your sending IP and domain on several.
- Your own bounce messages. They quote the reason and often link to the provider's help page for that block.
What damages reputation fastest
- Sending to a purchased or scraped list.
- Ignoring unsubscribe requests.
- A compromised website form or account that sends spam in your name.
- Sending to a very old list that you have not mailed for a year or more.
- Letting a contact form be abused as a relay.
Protect your forms with rate limits and bot checks, and keep API keys on the server. A leaked key can ruin months of reputation in an afternoon.
How to rebuild a poor reputation
- Stop the cause. Find out what went wrong before sending anything else.
- Clean the list. Remove hard bounces, non-openers over several months, and anyone who complained.
- Fix authentication and confirm alignment.
- Send low volume to your best recipients and increase slowly, as in the warm-up plan.
- Ask for delisting if you are on a blocklist, but only after the cause is fixed. Many lists expire entries automatically.
- Be patient. Recovery takes weeks, not hours.
Separate your streams
Use different subdomains for different kinds of mail so the reputation of one stream cannot sink another: app.yourdomain.com for transactional messages, news.yourdomain.com for newsletters, and the main domain for people-to-people mail. Each builds its own record while sharing a parent name.
How Mailions fits in
Each domain on Mailions has its own DKIM key, so the domain reputation it builds is its own and is not mixed with other customers. The sending API enforces per-key rate limits and a monthly recipient allowance, and keys are scoped to one domain, which limits the damage if a key leaks. Reputation still depends on what you send and to whom, so no service can promise it for you.