How ISPs Determine Email Deliverability

When your business sends an email, ISPs determine whether or not to deliver your message to their customers (your recipients).

Understanding how this process works can allow you to think strategically when sending, and take steps to improve your deliverability rate.

Siri is a computer program that has been thoughtfully developed, but occasionally makes a mistake. It’s up to me as the user to understand and alter my behavior to best utilize the power of Siri.

Like Siri, Internet Service Providers (ISPs) are programmed by people who write the rules, but it’s the computers who govern them.

Here’s how ISPs think to determine email deliverability.

Brand IP reputation

An IP address is the computer address from which your emails are sent.

Most people believe that the greatest influencer on your “deliverability” is your IP address. This is outdated thinking.

The ISPs realized that people sometimes switch email service providers and when they do, they get a new IP address.

Why does my IP reputation matter?

Your IP reputation matters because it’s a source of credibility that has been created between the IP and the ISP.

Your IP reputation matters, just not as much as your domain reputation. Here’s why– it’s hard for the ISPs to build a long-term history of a sender’s reputation if they switch providers a lot – just think, spammers could switch often and “restart” their reputation with every switch.

Because of this ISPs are weighing IP addresses less than they used to. You can share IPs with other senders or have an IP address all to yourself.

When should I have a dedicated IP?

According to Sendgrid, best practice dictates that if you’re sending more than 100k emails per month and at a steady cadence (at least 25k emails per week), you qualify for a dedicated IP.

When should I have a shared IP?

If you’re sending fewer than 100k emails per month and/or you send at an infrequent cadence (e.g. once per month), you should be on a shared IP.

Your Sending Domain Reputation

A sending domain is the human readable address from which your mail is sent.

It accompanies an IP address, which is the computer readable address from which your mail is sent.

Sending domains can be anything you want them to be though best practice is to either have them managed by your email service provider (shared; e.g. or have it make reference to your website (dedicated;

A sending domain reputation refers to the engagement and quality of your campaigns sent from a given sending domain over time.

Data like open rates and click-through rates are results ISPs refer to when determining the reputation of a sending domain.

Why should I care about Sending Domains?

Sending domains can be used across multiple IP addresses and multiple email service providers.

ISPs have adapted to weigh this piece of information the most heavily when deciding whether or not to trust your mail–essentially ISPs take the stance that you’re guilty until proven innocent.

They do this because businesses tend to want to have their emails look like they are coming from their website and, as a result, tend to change their sending domain less than other elements of their sending infrastructure.

It is crucial that you do not send to people who don’t engage with your emails because ISPs will believe that all email from your sending domain are not wanted, resulting in more of your emails landing in the spam or junk folder.

Your From Address Reputation

A From address is the reply-to email address you use for your campaigns.

Not to be confused with a From Label, which is just the name that comes through as the sender when an email is received.

For example, a campaign email might have the From Label as Mark | YourBrand, but the reply-to address is

It is crucial that you be as consistent as possible with your From address. The ISP algorithms are run by computers, so they see different from addresses as different, regardless of how similar they are.

If you are using a shared sending and/or a shared IP address, it is especially important that you are consistent because the ISPs look at your From address closely.

It is the only thing that allows them to tell how your emails are different from any other sender on the same sending domain or IP address.

How much does my From address reputation really matter?

The short answer is, it depends. Are you on a shared IP/domain combo?

If you are, it matters the most because it’s the only thing that makes your sending profile unique to you.

Ultimately ISPs want to identify different senders and understand how to treat their mail when it comes in.

The Content of the Message

Have you ever received an email from a Nigerian Prince asking for $10,000?

How about a dodgy looking email from your bank where the logo looks a bit off?

These emails are both examples of scams or phishing, which ISPs have to protect their customers from. It is your responsibility as a sender to prove that you are not scamming people.

Your emails must prove that the content is valuable to the receiver.

Here’s how you can prove your content is quality.

  • Subject line: If your subject line includes a phrase that seems like something people likely wouldn’t want to open (e.g. special characters, spammy words or a lot of caps), they will flag the message and send some of the emails to the spam filter.


  • Language in email body: If the content in your email seems like something people likely wouldn’t want to read (e.g. overly repeated text, spammy language, lots of special characters), they will flag the message and send some of the emails to the spam filter.
  • Unsubscribe link: When sending a campaign, you are legally required to allow people to unsubscribe from your list.
  • If you do not include an unsubscribe link, you run the risk of people reporting your email as spam instead of unsubscribing, which is a lot worse.
  • ISPs count spam reports very heavily and will penalize your future campaign sends by assuming your messages will be spam and putting them in the spam filter proactively.
  • Image/text ratio: ISPs cannot read the text included within an image.
  • As a result of this, they have to assume that images include text that could be phishing attempts or “bad” content.
  • If your emails are predominantly images, ISPs won’t trust your content and will send more of it to the spam filter.
  • To get around this, include text that describes each of your images to your recipients and, at the very least, include alt-text for each of your images.
  • Link domains: Do the destination URLs in your link point to either the domain in your from address or sending domain?
  • If not, ISPs may believe that you are affiliate marketers (getting paid to send emails to your recipients peddling other people’s products), which are generally emails that people do not want to receive.
  • Make sure the domains of the links in your email are consistent – this is proof that you are a legitimate brand and gives ISPs more confidence that you are sending good emails.


Looking for More Information?

Back to Blog Home
Get email marketing insights delivered straight to your inbox.
*By entering your email address and clicking Subscribe, you consent to receive marketing emails (such as newsletters, blog posts, webinars, event invitations and new product updates) from Klaviyo from time to time. You can unsubscribe at any time by clicking on the “Unsubscribe” link at the bottom of our emails. For more information on how we process your personal information and what rights you have in this respect, please see our Privacy Policy.
Own your data.
Own your growth.
© 2022 Klaviyo. All rights reserved. Klaviyo and the Klaviyo logo are trademarks or registered trademarks of Klaviyo, Inc. or its affiliates.
Terms and Privacy Manage Cookies