IP Reputation

IP reputation is the trust score mailbox providers and blocklist operators assign to the IP address that sends an email, based on the volume, complaints and spam seen from it. A poor IP reputation can cause throttling, spam placement or outright rejection.

Deliverability & Email InfrastructureUpdated September 30, 2026

In short

IP reputation is the track record of the server address your mail leaves from, shared by everyone who sends through that address.

Key points

  1. Gmail tracks volume, feedback and limits per domain and per IP, and the IP quota is shared by every sender using that address [1].
  2. On a Shared IP vs. Dedicated IP, other customers' behavior affects your delivery; a dedicated IP isolates you but needs steady volume to build history.
  3. Spamhaus publishes IP blocklists such as the SBL, and many receivers reject mail from listed addresses at connection time [2].
  4. Microsoft's Smart Network Data Services (SNDS) shows IP owners how their traffic to Outlook.com is rated [3].
  5. Valid Reverse DNS (PTR Record) is a baseline requirement; Gmail expects forward and reverse DNS for all sending IPs [1].
  6. Senders using Gmail or Microsoft 365 mailboxes send from the provider's well-managed IPs, so Domain Reputation matters more for them.

How IP reputation is measured

Every connection to a receiving mail server comes from an IP address, and the receiver can judge that address before reading a single message. Signals include how much mail the IP sends, how steady that volume is, the share of messages that bounce or hit a Spam Trap, and the Spam Complaint Rate among recipients. Receivers also consult public and private blocklists. Spamhaus runs several: the SBL lists IPs tied to spam operations, and other lists cover compromised machines and address ranges that should not send mail directly [2]. Gmail notes that it tracks limits both per domain and per IP, and that an IP's quota is shared across all domains sending from it [1]. A bad score often shows up first as temporary deferrals rather than bounces.

Shared versus dedicated IPs

Most Email Service Provider (ESP) platforms put smaller senders on pools of shared IPs. The provider manages those pools carefully, and the combined volume keeps their reputation stable, but one bad actor can drag down everyone on the address. A dedicated IP gives a sender full control and full responsibility. It starts with no history, so it must go through Email Warmup, and it needs enough steady volume, typically tens of thousands of messages a month, for receivers to form a view. Sending too little mail through a dedicated IP makes each spike look suspicious. For Cold Email sent from a regular business mailbox, neither option applies: messages leave from the mailbox provider's servers, whose IP reputation the provider maintains.

Monitoring and recovering

Senders with their own IPs should register them with the tools the major providers offer. Microsoft's SNDS reports traffic volume, complaint rates and trap hits for IPs sending to Outlook.com, with a color-coded status [3]. Google Postmaster Tools reports spam rate and compliance for Gmail traffic. Checking Spamhaus and other lists after any deliverability drop helps spot an Email Blocklist listing early [2]. If a listing or poor score appears, the fix is to find the cause, usually a compromised account, a bad list or a volume spike, stop it, and then request removal where the list allows it. Afterwards, slow down and send only to engaged recipients until the numbers recover, following Gmail's advice to increase volume gradually [1].

Sources
  1. Email sender guidelines — Gmail Help
  2. Spamhaus Blocklist (SBL) — Spamhaus
  3. Smart Network Data Services — Microsoft
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Related terms

Sender Reputation

Sender reputation is how much a mailbox provider trusts your mail, and it is earned or lost through how recipients react to what you send.

Domain Reputation

Domain reputation is the track record your domain name builds with mailbox providers, and it travels with you wherever you send from.

Shared IP vs. Dedicated IP

Shared IPs pool reputation across senders, while dedicated IPs make you solely responsible for your own.

Email Blocklist

An email blocklist flags IPs or domains linked to spam so receivers can reject their mail.

Reverse DNS (PTR Record)

Reverse DNS lets a receiver look up the name behind a sending IP address, and mail from IPs without it is often rejected.

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