We have put together an in-depth overview of sender reputation to help you understand what it is and how it affects your marketing results.


Sender reputation also referred to as domain reputation or sender score, is a rating system that an email service provider assigns to each email sender’s domain and is used to determine whether an email will be blocked, delivered to spam, or delivered to the inbox.



  1. Evolution of sender reputation
  2. Impact of sender reputation
    • Deliverability
    • Reputation levels
  3. Sender reputation key factors
    • Overview
    • Domain or URL reputation
    • IP address reputation
    • Autonomous System Number (ASN) reputation
    • Sender history
    • Hard and soft bounces
    • Email user actions
  4. Sender Identification and Authentication
    • Overview
    • Sender Policy FrameWork (SPF)
    • DomainKeys Identified Mail (DKIM) signatures
    • Domain-based Message Authentication Reporting and Conformance (DMARC)

Evolution of sender reputation

The first-ever spam-prevention system was the Mail Abuse Prevention System (MAPS) created in 1990 (see History of email spam), which kept records of IP addresses that had sent spam.

In 2003, the first proposal for what would become the Sender Policy Framework (SPF) was introduced, still focused exclusively on IP address lists.

The following year saw the introduction of DomainKeys Identified Mail (DKIM) to prevent tampering with mail in-transit. This allowed email providers to verify the sender’s validity with SPF records and the integrity of the email with DKIM.

In 2006, Amazon Web Services was launched, and the growth of public cloud infrastructure grew rapidly, reducing the barrier to creating remote email servers and rapidly changing IP addresses with the goal of fooling existing spam-protection measures.

In an evolving effort to defend against spoofed email addresses, Domain-based Message Authentication Reporting and Conformance (DMARC) was introduced in 2012. DMARC is a combination of SPF and DKIM to prove both the sender’s validity and the integrity of the individual email.


Impact of sender reputation


Sender reputation is the highest factor in determining email deliverability. Google states that it uses hundreds of signals to determine deliverability, but research has shown that 82% of deliverability errors are due to sender reputation.

Reputation levels

Google uses the following four sender reputation levels to rank each sender:

Bad A history of sending an enormously high volume of spam. Mail coming from this entity will almost always be rejected at SMTP or marked as spam.
Low Known to send a considerable volume of spam regularly. Mail from this sender will likely be marked as spam.
Medium/fair Known to send good email, but prone to sending a low volume of spam intermittently. Most of the email from this entity will have a fair deliverability rate, except when there is a notable increase in spam levels.
High Has a good track record of a very low spam rate and complies with Gmail’s sender guidelines. Mail will rarely be marked by the spam filter.

Google identifies spam as email that Gmail’s spam filter detects and also email that is reported by Gmail users as spam.[1]


Sender reputation key factors


Sender reputation is a concept used to separate valid email senders from spammers, meaning that it is a comparison of the behavior of spammers to that of everyone else. Spammers have the poorest reputation score, and the more behavior an email sender exhibits that matches a spammer, the lower that sender’s score will be.

Domain or URL reputation

The sending domain of the email is the unique identifier located after the “@” sign in an email. Each email sent from a specific domain contributes to that domain’s reputation.

IP address reputation

Where a domain is a unique identifier, an IP address can be the host for numerous different companies. For example, in a hosted cloud environment, a single IP address can be used to send email for multiple different domains. Spam traffic from any of the domains can negatively impact the reputation of the IP address.

Autonomous System Number (ASN) reputation

In the same way that an IP address can host multiple domains, an ASN is assigned to a collection of IP networks. Because the ASN is two levels removed from a company-specific domain, it is unclear what the level of impact is on overall sender reputation, and this is largely out of the control of the email sender.

Sender history

A multi-year history of gradually increasing email list size with consistent monthly send frequency tells the story of a legitimate sender who is growing a list over time. A brand new account that immediately sends a high volume of email is a common spam tactic that is likely to be blocked. Even an existing account that suddenly sends an abnormally high volume of email traffic is considered suspicious activity that email providers must take into consideration when deciding to deliver or block those emails.

Hard and soft bounces

A bounce is a non-deliverable email. Hard bounces are invalid email addresses where soft bounces are valid emails that were undeliverable due to a problem with the receiving mail server (e.g. mailbox is full). Email spammers will typically have a high percentage of invalid email addresses and any bulk email sender should strive to reduce hard and soft bounce rates to as close to 0% as possible as any hard or soft bounces will negatively affect the sender’s reputation.

Email user actions

Upon receiving an email, a user can take both positive and negative actions. These actions affect future emails from that sender to the individual user, and if enough users take particular actions, these actions will count towards an upgrade or downgrade of sender reputation.

Positive actions include but are not limited to the number of opens, viewing time, number of clicks on the email content, number of positive replies, number of forwards, marking as important.

Negative actions include but are not limited to not opening the email, not clicking on content, unsubscribing, replying with unsubscribe or removal language, deleting, marking as spam.


Sender identification and authentication


Since 2003, various efforts have been put in place to combat email spam that center on identifying the sender and verifying that emails have not been tampered with. Spammers inherently do not want to be identified because their activities are questionable at best, so bulk email senders who are willing to identify themselves can demonstrate their trustworthiness and allow the email provider to accurately track all of their email histories. Identification is a big part of sender reputation, and every activity performed under a sender’s domain and IP addresses affect reputation positively or negatively.

Sender Policy Framework (SPF)

An SPF list specifies the valid IP address through which a company’s emails can be sent. Any IP address not published in the list that is sending emails could be a spammer that is spoofing (or pretending to be) a valid address.

DomainKeys Identified Mail (DKIM) signatures

DKIM provides authentication for an email to verify that it was sent from a specific domain and was not tampered with during delivery.

Domain-based Message Authentication Reporting and Conformance (DMARC)

DMARC is an email validation technique to prevent a type of email spoofing used in phishing and email spam where a fraudulent person forges the sender address so that the email appears to originate from legitimate organizations. DMARC combines SPF and DKIM authentication in order to verify both that the sender is valid and that the sender’s valid identification was used. This is valuable to email providers in preventing spoofed email addresses.