Sender Reputation Explained: IP, Domain and Warm-up

Every mailbox provider keeps a record of your earlier emails and of how its users responded to them. That record is your sender reputation. It helps the provider decide whether to accept your next email and whether to put it in the inbox or the spam folder. Microsoft says: "Deliverability to Outlook.com is based on your reputation."

You cannot set your reputation or buy it. You build it slowly through the emails you send and how people respond to them, but you can damage it quickly.

This guide covers:

1. What sender reputation is

2. IP reputation and domain reputation

3. What builds and what damages reputation

4. Your sending service keeps a record too

5. Warming up a new IP address or domain

6. How to see your reputation

7. Repairing a damaged reputation

8. Reputation tools in AcelleMail

It ends with the most common mistakes and a one-screen summary.

1. What sender reputation is#

Google explains sender reputation in its help for Postmaster Tools: "Reputation is a rating of the quality of domains and IP addresses used to send email, and is determined by the sending behavior of a domain or IP address." It adds: "Messages from senders with a high reputation are more likely to be delivered to Gmail’s Inbox instead of to Spam."

Gmail shows four ratings in Postmaster Tools:

Rating	Google's description
High	"History of very low spam rates, and complies with Gmail's sender guidelines."
Medium	"History of sending legitimate email, but occasionally sends spam."
Low	"History of sending a significant volume of spam regularly."
Bad	"History of sending a high volume of spam regularly."

There is no single score shared by every provider. Gmail, Microsoft and Yahoo each keep their own record. They base it on the emails they receive from you and how their users respond. So your reputation can be good with one provider and weaker with another. In Microsoft's words, "Reputation is always the responsibility of the sender."

2. IP reputation and domain reputation#

Providers usually track two kinds of reputation for your email:

• IP reputation belongs to the IP address of the server that sends your email.
• Domain reputation belongs to your domain. In Gmail's Postmaster Tools, it covers the domain that signs the email with DKIM, or the SPF domain when there is no DKIM signature.

Yahoo says: "Each IP and DKIM domain has a reputation, which can impact the delivery of your email."

	IP reputation	Domain reputation
What it belongs to	The sending IP address	Your domain, as used in DKIM or SPF
Who shares it	Everyone who sends from the same IP address	Only email authenticated with your domain
When you change IP address or service	You leave the old IP reputation behind	It stays with your domain

When you use a shared IP address, emails from other senders also affect its reputation. Google explains in its email sender guidelines: "The IP address quota is shared for all senders that use that IP address. When the IP address hits its quota, all domains that send from that IP address stop sending emails."

This does not mean you need a dedicated IP address. Amazon SES advises: "If you don't plan to send large volumes of email on a regular and predictable basis, we recommend that you use shared IP addresses." Shared IP addresses are ready to use immediately. Dedicated IP addresses keep your reputation apart from other customers, but they need setup and a warm-up. Dedicated vs Shared Sending IP compares the two.

It also helps to keep different types of email separate. Yahoo advises against sending bulk or marketing email from the same IP addresses as personal email, transactional email and alerts. Transactional vs Marketing Email explains why.

3. What builds and what damages reputation#

Your reputation depends on how people respond to your emails. A few signals have the greatest effect.

• Complaints. Every spam report counts against you. Gmail asks every sender to keep the spam rate reported in Postmaster Tools below 0.3%, and Yahoo sets the same limit. Email Marketing Metrics Explained shows how the rate is calculated, and Spam Filters Explained shows how readers' reports train the filters.
• Email nobody asked for. Google asks senders to avoid practices such as "Don't purchase email addresses from other companies." and "Don't send messages to people who didn't sign up to get messages from you."
• Bounces and spam traps. Sending emails to addresses that do not exist or to spam traps suggests that you have not kept your list clean. Email List Quality Explained covers both.
• Sudden jumps in volume. Amazon SES says that its sending quotas help "decrease the likelihood that email providers block your emails because of sudden, unexpected spikes in your email sending volume or rate." Section 5 explains how to increase your volume gradually.
• Authentication. Gmail's reputation dashboards only include emails authenticated with DKIM or SPF. This means emails without authentication do not count towards a domain's reputation in Postmaster Tools. That does not make them any safer to send. Email Deliverability Explained explains the records.

4. Your sending service keeps a record too#

Mailbox providers are not the only ones that watch your email. Your sending service, which Email Sending Explained describes, also watches your account because your emails affect its IP addresses and its own reputation. Amazon SES publishes the levels it uses to decide when to take action:

Rate	Amazon SES's advice	Account placed under review	Sending may be paused
Bounce rate	below 2%	5% or more	10% or more
Complaint rate	below 0.1%	0.1% or more	0.5% or more

So a single bad list, for example an old or purchased one, can put the whole account at risk. If you run AcelleMail for several customers, the customers who use your shared sending servers all send through the same sending service accounts. That is why AcelleMail can watch each customer's bounces and complaints (section 8).

5. Warming up a new IP address or domain#

A new IP address or domain has no history, so providers are careful with senders they do not know yet. Amazon SES explains: "Email providers are less likely to accept mail from new IP addresses that have little or no history." The solution is to increase your sending volume step by step. SendGrid describes this process as follows: when you add a dedicated IP address, "gradually increase the volume of email messages sent through the IP address".

Google's sender guidelines name three factors that affect how quickly you can increase your volume:

• The amount of email. The more email you send, the more slowly you should increase the volume.
• How often you send. You can increase the volume more quickly when you send daily instead of weekly.
• Recipient feedback. Send only to people who subscribed, and give them a way to unsubscribe.

Google also says: "As you gradually increase your sending email volume, you should monitor your email delivery with Postmaster Tools."

Sending services can handle part of the work. Amazon SES can warm up standard dedicated IP addresses for you. Its warm-up percentage "steadily increases over 45 days, independently from your sending volume". Afterwards, Amazon SES says that you "should send around 1,000 emails every day to each email provider that you want to maintain a positive reputation with". That advice is for a dedicated IP address, which is one more reason why small senders do better on shared IP addresses (section 2).

A new dedicated IP address needs a warm-up, as does a new domain or subdomain. An IP address that has been inactive also needs one. SendGrid advises warming it up again when no email has gone through it "in more than 30 days". The shared IP addresses of a sending service are already in use, so you do not need to warm them up yourself. However, a new domain using those IP addresses still starts without a sending history. Start by sending to contacts who are most likely to open your emails. IP Warmup Schedule and Domain Warmup give example schedules.

6. How to see your reputation#

Each provider shows only what it sees of your email. No provider shows the full picture.

• Gmail. Postmaster Tools has dashboards for the spam rate, IP reputation and domain reputation. It also shows your compliance status against Gmail's sender requirements. Gmail Postmaster Tools Walkthrough shows how to set it up.
• Outlook.com. Microsoft's Smart Network Data Services (SNDS) provides data about the IP addresses you control. It includes the Junk Email Reporting Program, which sends you a report when users mark your messages as junk. A new SNDS portal is replacing the old site, according to an announcement updated on 22 September 2026. Microsoft SNDS Walkthrough explains the setup.
• Your own reports. You can check each campaign's bounces and complaints after every send. These can help you assess your sending reputation.

These tools do not combine their results into one score. Each tool shows only what its own provider sees, so read the results one by one.

7. Repairing a damaged reputation#

When your numbers drop, find the cause before you change anything. Common causes include a purchased or very old list, a sudden increase in volume, missing authentication, or a type of email that people did not expect.

Then change how you send while your reputation recovers. Stop sending to people who have not engaged for a long time, clean the list, and fix the authentication. Send fewer emails and send only to your most engaged contacts until the complaint and bounce rates come down. This process takes time. Microsoft says: "Building & maintaining good reputation is a long-term proposition."

Avoid moving to a new IP address or domain to escape the problem. The new one starts without a sending history. If the cause is still there, the same problem can happen again.

8. Reputation tools in AcelleMail#

Warm-up strategies#

A warm-up strategy tells AcelleMail how many emails a sending server may send each day during its warm-up. Open Sending → Warmup strategies to create one.

1. Preset and Growth strategy: start with Balanced, Cautious or Aggressive, and choose Linear or Exponential growth.
2. Starting volume: how many emails the server may send on day 1.
3. Daily increment: how many emails the server adds each day with linear growth.
4. Limit type and Per day cap: what ends the warm-up, for example a daily cap.

The Strategy preview on the right shows the plan as a chart. In this example, the Cautious preset starts at 15 emails a day, adds 8 a day and stops growing at 500 a day. If the server does not send at weekends, the preview estimates that the full warm-up will finish on calendar day 86.

AcelleMail applies the starting volume, daily increase, cap and weekend setting to the campaigns and automations that the server sends. It counts the plan by calendar day for the whole server, not separately for each mailbox provider. The warm-up starts the first time a campaign uses the server. After the last planned day, the server sends at full speed.

To use a strategy, open the sending server. In its Warmup card, choose the strategy under Select strategy and click Assign strategy. The card explains: "Ramp this server's sending volume up gradually so mailbox providers build trust before you send at full speed."

Sending health, for administrators#

If you run AcelleMail for several customers, Sending health watches each customer's hard bounces and complaints. Administrators set it up under Settings → Sending health.

1. Mode: Off, Monitor or Enforce. In Monitor mode, "Rates are measured and administrators are told who would be suspended. Customers are neither warned nor suspended." In Enforce mode, "Customers are warned and suspended automatically when they reach the marks."
2. Hard bounces: by default, a warning at 2% and a suspension at 5%, based on a customer's last 2,000 emails, with at least 20 bounces.
3. Complaints: by default, a warning at 0.1% and a suspension at 0.3%, based on the last 20,000 emails, with at least 15 complaints.

These defaults follow the levels in sections 3 and 4. The warnings start at Amazon SES's advice: 2% bounces and 0.1% complaints. In Enforce mode, a customer is suspended at 5% hard bounces or 0.3% complaints. Amazon SES places an account under review at 5% bounces, and Gmail and Yahoo ask senders to keep the spam rate below 0.3%. Both levels are lower than the 10% and 0.5% at which Amazon SES may pause the shared account.

9. Common mistakes#

What you see	Why it is a problem	What to do
You move a large list to a new dedicated IP address on the first day.	A new IP address has no history, and providers are less likely to accept its email.	Warm it up, starting with your most engaged contacts.
You buy a dedicated IP address for a small, irregular newsletter.	Amazon SES recommends shared IP addresses unless you send large volumes regularly.	Stay on shared IP addresses.
Marketing email and order emails come from the same IP address and domain.	Complaints about the marketing email can then affect the order emails too.	Keep the two kinds of email apart, as Yahoo advises.
You ignore complaints because the rate looks small.	Gmail and Yahoo want the spam rate below 0.3%, and Amazon SES reviews an account from 0.1%.	Watch the complaints for each campaign.
One customer on your platform imports an old list.	If customers share a sending server, that customer's bounces and complaints count against the whole sending account.	Watch each customer's rates with Sending health before the sending service acts.
You switch to a new domain after a reputation problem.	The new domain starts without history, and the cause is still there.	Fix the cause first, then rebuild slowly.
You stop sending for months, then email everyone at once.	SendGrid advises a new warm-up when an IP address has sent nothing for more than 30 days.	Increase the volume gradually again.

Summary#

1. Sender reputation is each provider's record of your earlier emails and of how its users responded to them. Gmail rates it from High to Bad.
2. IP reputation belongs to the sending IP address and is shared by everyone using a shared IP address. Domain reputation stays with your domain.
3. Complaints, unwanted emails, bounces and sudden jumps in volume damage it. Wanted, authenticated emails help build it.
4. Your sending service keeps a record too. Amazon SES reviews an account at 5% bounces or 0.1% complaints.
5. Warm up a new IP address or domain slowly and steadily, starting with engaged contacts.
6. Check your standing in Gmail's Postmaster Tools and Microsoft's SNDS.
7. Repair takes time. Fix the cause, then rebuild slowly.
8. In AcelleMail, use a warm-up strategy for each new server, and Sending health if you run AcelleMail for several customers.

Terms used here are defined in the Email Marketing Glossary.

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