What sender reputation is
Every time an email arrives, the receiving side has to decide: inbox, spam folder or refuse it. The services that run inboxes are called mailbox providers: the big free-mail services, the companies that host work email, and organisations that run their own mail servers.
A mailbox provider cannot ask a sender whether they can be trusted. So it looks at the sender's history. Sender reputation is the trust a provider has built up about a sender from everything it has seen before. It works a little like a credit score, with two differences: every provider keeps its own score, and none of them publishes the formula.
There are two main kinds:
IP reputation belongs to the IP address of the server that hands the message over. Many email services send mail for lots of customers from the same shared IP addresses, so the behaviour of other customers can affect you. Very large senders often have their own (dedicated) IP addresses.
Domain reputation belongs to your domain: the one in the From address, in the DKIM signature and in the links. It travels with you when you change email service. Because IP addresses are often shared and change, domain reputation carries a lot of weight today.
Some large providers offer free dashboards that show senders how they are rated. Google's Postmaster Tools, for example, shows how Gmail sees a domain.
What mailbox providers look at
Providers publish guidelines, but not their filters. Nobody outside knows exactly how much each signal counts. The table sums up what the providers' own guidelines and long experience agree on.
Signal | What it means | Why it matters |
|---|---|---|
Authentication | SPF, DKIM and DMARC pass and match the From domain | Proves the mail really comes from you, so reputation can be attached to your domain at all |
Spam complaints | Recipients click "Report spam" | One of the strongest negative signals there is |
Bounces | Messages to addresses that do not exist | Shows a list that is old, bought or guessed |
Engagement | Replies, reading, moving a message out of spam; or deleting without reading | Shows whether people want your mail |
Spam traps | Addresses that never belonged to a real person, or old addresses that were closed and later reused as traps | Hitting them shows a list that was not collected with permission or not cleaned |
Volume pattern | Steady, slowly growing volume, or sudden bursts | Spammers often send big bursts from fresh domains |
Content and links | Links to domains with a bad name, misleading wording | Usually a smaller signal than the ones above |
Complaints have a public benchmark. Google's sender guidelines ask senders to keep the spam rate shown in Postmaster Tools below 0.1 percent and never to reach 0.3 percent. Other large providers have published similar rules.
Worked example 1: reading the signals
Fabrikam, a fictional office furniture company, sent 2,000 sales emails last week. 120 of them bounced because the addresses did not exist, 9 recipients reported the email as spam and 64 replied. All figures are invented.
Bounce rate: 120 ÷ 2,000 = 6 percent of sent emails.
Delivered: 2,000 − 120 = 1,880.
Complaint rate: 9 ÷ 1,880 = 0.48 percent of delivered emails.
Reply rate: 64 ÷ 1,880 = 3.4 percent of delivered emails.
The replies look fine, but they do not cancel out the rest. The complaint rate is far above the 0.1 percent Google asks for, and a bounce rate of 6 percent suggests a list that was never checked. (As a rough illustration only, many senders aim to keep bounces well under 2 percent; there is no official line.) Fabrikam should stop, clean the list and find out why so many people felt the email was unwanted.
Why a new domain or mailbox starts slowly
A domain registered last week has no history at all. Providers are careful with unknown senders, because spammers register fresh domains, send as much as they can, and move on when the domain is blocked. So a new sender is watched closely, and its mail may be slowed down or filtered until there is enough evidence that recipients want it.
Google's guidelines say it plainly: increase sending volume slowly, start with a low volume to engaged recipients, and avoid sending in bursts. Building up volume step by step like this is called warm-up.
Worked example 2: a warm-up plan
Tailspin Toys, a fictional toy wholesaler, sets up a new mailbox for its sales team. This is a made-up plan to show the idea, not a rule from any provider.
Week | Emails per working day | Who gets them |
|---|---|---|
1 | 10 to 20 | Existing customers and contacts who know the team and are likely to answer |
2 | 20 to 30 | The same, plus new contacts who fit well and were checked carefully |
3 | 30 to 50 | New contacts, only if week 2 had almost no bounces and no complaints |
4 and later | Slowly more | Only while bounces and complaints stay low |
The rule behind the table matters more than the numbers: only go up when the last step was clean. If bounces or complaints rise, stay at the same level or go back a step.
Sending limits
Every provider limits how much one account may send, for example per day, per hour or per message. New and trial accounts often get lower limits. Going over a limit usually means a temporary block on sending. The numbers differ and change over time, so look them up on the provider's own current help page, for example Gmail sending limits in Google Workspace.
The receiving side has limits too. A provider that does not know a sender yet may accept only a few messages at a time and answer the rest with "try again later". This shows up as temporary (soft) bounces, which the bounce exercise explains.
A word on automatic warm-up services
Some services offer "automatic warm-up": your mailbox exchanges messages with many other mailboxes in a network, which open them, reply and move them out of spam. The goal is to create engagement that looks real. Mailbox providers have not said that they reward this, and it may break the terms of use of some email services. The reliable way to build a reputation is the slow one: real emails to real people who want them.
A separate domain for outreach?
Many companies send sales emails from a second domain, for example northwind-team.example next to northwind.example. That has real advantages and real drawbacks.
For | Against |
|---|---|
Problems with sales email do not hit the main domain, which also sends invoices, support answers and password resets | The new domain has no reputation and needs its own warm-up |
Reputation and reports for sales email are easy to see on their own | A lookalike domain can seem less trustworthy, even like phishing, if recipients do not recognise it |
The main domain can keep a strict DMARC policy without risk to sales tools | It needs its own SPF, DKIM and DMARC records and a website or a redirect to the main one |
It changes nothing about the law: the sender and the duties stay the same |
A separate domain separates risk; it does not remove it. If a company needs a new domain every few months because the old one keeps getting blocked, the problem is the emails and the list, not the domain.
Blocklists
A blocklist is a list of IP addresses or domains that have sent spam. Independent organisations publish them, and mail servers check them through DNS before accepting a message, which is why they are also called DNS-based blocklists (DNSBL).
How you get listed: hitting spam traps, many complaints, or a hacked account that sends spam in your name.
How to check: most blocklists have a public lookup page where you type in an IP address or domain.
How to get off: fix the cause first, then follow the list's own removal procedure. Some listings expire on their own once the spam stops. Be suspicious of anyone who offers paid removal from a list they do not run.
Blocklists differ a lot. Some are widely used by large providers; others are small and have little effect.
How to recover a damaged reputation
Stop or slow down. Every further email to a bad list makes it worse.
Find the cause. Check authentication, bounce reports and complaint rates, and ask what changed: a new list, a new tool, a hacked account?
Fix authentication if SPF, DKIM or DMARC fail or do not match the From domain.
Clean the list. Remove hard bounces, bought or very old contacts, and people who never react.
Restart small with the people most likely to want your mail, and ramp up again like a warm-up.
Ask for removal from any blocklist once the cause is fixed.
Be patient. Reputation is history. It improves as good mail builds up, which usually takes weeks, not days.
Common mistakes
Sending the whole list on day one from a new domain or mailbox.
Believing authentication is enough. SPF, DKIM and DMARC prove who you are. They do not prove that people want your mail.
Judging success by opens. Open tracking is unreliable, as the outreach metrics exercise shows. Replies, bounces and complaints say more.
Burning domains. Switching to a new domain whenever the old one has problems repeats the cause every time.
Ignoring soft bounces. Many "try again later" answers are often the first sign that a provider is slowing you down.
Hiding the way out. If unsubscribing is hard, people press "Report spam" instead, which hurts much more.
Buying lists. They bring bounces, spam traps and complaints, and in many countries they are also a legal problem.
Check yourself
1.What is the difference between IP reputation and domain reputation?
2.Which signal is one of the strongest negative signals for a sender?
3.Why should a brand-new domain not send 1,000 emails on its first day?
4.Week 2 of a warm-up had several bounces and two spam complaints. What should the team do in week 3?
5.Which statements about a separate domain for outreach are true? Pick two.
More than one answer is right.
6.A domain is on a blocklist because a hacked account sent spam. What comes first?
7.Litware sent 1,000 emails, 40 bounced and 5 people reported spam. What is the complaint rate of delivered emails?