Understanding inbox placement

Key findings on inbox placement and measurement

Measuring deliverability and inbox placement

A variety of metrics provide insights into email deliverability, but which one are you using to find out if emails make it to the inbox?

When we asked 1,100 senders to select the metrics and methods they actively use to measure email deliverability, these were the top five responses:

1. Open and click rates (57.4%)

2. Delivery rate (53.1%)

3. Bounce rates (46.5%)

4. Unsubscribe rate (39.5%)

5. Spam complaints (32%)

Email engagement metrics, such as opens and clicks, are important to monitor. You want to maintain good email engagement to achieve inbox placement. When your contacts open and click, it’s a sign to mailbox providers that your emails are wanted. A sudden drop in typical open and click rates may also indicate there are deliverability problems.

Conversely, a high unsubscribe rate can signal that your emails are unwanted. While it’s normal to have contacts unsubscribe, if the rate increases, you may have an issue with the sending frequency or relevancy of your emails.

Getting marked as spam is one of the fastest ways to end up in the spam folder, so it’s surprising that less than one-third (32%) of senders say they are actively monitoring the spam complaint rate. This seems to be an undervalued metric, especially considering the sender requirement of a 0.3% threshold for spam complaints.

53% of senders actively monitor their delivery rate. That’s the second most-common metric among all respondents. However, the delivery rate does not measure what most senders think it does.

The truth about your email delivery rate

In somewhat of a twist, we threw a “quiz question” into the State of email deliverability 2025. We wanted to know how senders define the email delivery rate. What do they think it measures? 11.6% could admit they didn’t know. The rest were willing to make a guess:

Shockingly, this means almost 88% of senders in our survey could not correctly identify what the delivery rate measures. This is perhaps the biggest misconception in email deliverability. It’s time to clear things up.

The delivery rate metric measures the percentage of emails accepted by a receiving mail server and delivered to any folder. When emails land in spam, it still counts towards the delivery rate. Those emails were delivered... to the junk folder. The only messages that don’t count toward the delivery rate are those that bounce or get rejected by the receiving mail server.

So, if you have a delivery rate of 98%, you know that most of your emails were delivered. But what you don’t know is if you were able to avoid spam.

Senders with larger email volumes were more likely to answer the delivery rate question correctly. Nevertheless, more than 3/4 of those senders still got it wrong. Less than a quarter of the highest volume senders (24%) could correctly define the delivery rate.

This may leave you wondering how you can measure where your emails land. Is there a way to know if you’ll reach the inbox or get filtered into spam? There is... it’s called the inbox placement rate and the only way to uncover it is with a specific type of email testing. Unlike the delivery rate, inbox placement testing tells you what happens to emails that get delivered.

Here’s how the inbox placement rate is calculated:

Inbox placement rate % = (# of messages in the inbox ÷ # of messages delivered) X 100

How inbox placement testing works

The inbox placement rate isn’t a metric that shows up in your typical email analytics. That’s probably why so few senders in our survey are using it.

Only 13.3% of respondents use inbox placement reports to measure email deliverability. That percentage does increase based on email send volumes. But even among organizations sending more than one million emails per month, only about 1 in 4 conduct inbox placement testing.

Inbox placement testing is also known as “seed testing.” That’s because the process uses seed mailboxes from major providers to find out where your messages end up. In the end, you get a report providing visibility into inbox placement.

Here’s how inbox placement testing works:

If necessary, you can retest your email after adjusting. Results from inbox placement reports can reveal issues like blocklisting and authentication failures. They’ll also help you optimize future sends for improved deliverability.

Deliverability challenges: Quick tips to avoid spam

Inbox placement testing is the best way to help you take on the main challenge in email deliverability – avoiding the spam folder.

When we asked senders to select their top three email deliverability issues, staying out of spam emerged as the clear winner. 48% of all senders cited staying out of spam as a top challenge. That’s followed by maintaining list hygiene at nearly 34% and reducing bounce rates at around 28%.

Higher volume senders were less likely to cite avoiding spam as a challenge and more likely to have challenges with low subscriber engagement and list hygiene than smaller senders.

Your customers aren’t too happy when emails land in spam either. That’s especially true if it’s a message they’re anticipating. When we surveyed consumers for the Sinch Mailgun report, Email and the customer experience, we found many people have a negative reaction when a brand’s emails land in spam.

While some consumers won’t care or don’t notice if your emails land in spam, that’s not necessarily a good thing. It suggests they’re so uninterested in what you’re sending that it doesn’t matter where your messages end up.

How to stay out of spam

Here’s some essential advice to prevent your emails from going to your contacts’ spam folders. These are basic tips, but they’ll go a long way towards improving inbox placement.

1. Set up email authentication: This verifies your identity as the sender and supports your reputation with mailbox providers.

2. Obtain consent first: Before you add a contact to your email marketing list, be sure to get permission. A double opt-in process helps confirm intent to subscribe.

3. Make it easy to unsubscribe: When contacts can’t opt out, they mark you as spam instead. Follow RFC 8058 for one-click unsubscribe and honor requests as soon as possible.

4. Monitor user complaints: Keep a close eye on your spam complaint rate. Strive to keep it under 0.1% and only send emails your contacts want to receive.

5. Comply with regulations: Make sure you follow key laws including the EU’s GDPR, the CAN-SPAM Act, and other data privacy regulations.

6. Keep your list clean: Manage your contacts by keeping invalid email addresses and spam traps off your lists. Segment or remove unengaged subscribers.

7. Conduct inbox placement testing: Know if you’re likely to be filtered into spam before you send by reviewing inbox placement reports from seed mailboxes.

To sum it all up – if you want to avoid the spam folder, don’t act like a spammer.

Email blocklist monitoring

What’s worse than being filtered into spam? Ending up on an email blocklist. Blocklists are directories of IP addresses and/or domains that get flagged as sources of suspicious behavior, malicious content, or spam.

There are many reasons why you could be placed on a blocklist, sometimes it’s your fault and sometimes it’s not. Either way, it’s a situation you’ll want to remedy quickly. Here are some common explanations for being blocklisted:

• You’ve been emailing people without their consent.

• You’re on a shared IP along with senders who have a bad reputation.

• You ramped up sending volume without warming a new domain or IP address.

• Your spam complaint rate got too high.

• You sent an email to a spam trap after buying a list of contacts.

• You violated consumer privacy laws.

• You lack proper email authentication.

While blocklist providers such as Spamhaus and Barracuda are willing to work with reputable senders, they aren’t in the habit of alerting anyone about landing on their lists. Why would they want to tip off real spammers and scammers?

Blocklist monitoring is an email deliverability practice that alerts senders if their domain or sending IP ends up on one of the “naughty lists.” Monitoring blocklists allows you to address the problem before it negatively impacts your business.

Our survey found 32.7% of senders are monitoring blocklists but more than half are not and 14.5% are unsure.

A blocklist monitoring service could save your organization from some serious headaches. Imagine what would happen if transactional emails such as order confirmations and password resets were blocked by a major mailbox provider. What if a Black Friday email campaign was blocked from reaching consumers?

“Landing on a blocklist can impact your bottom line as well as your future sending. By proactively monitoring and addressing blocklist issues as they arise, you can safeguard your reputation, maintain customer trust, and ensure your messages aren’t automatically flagged as spam.”

Kate Nowrouzi

VP of Deliverability and Product Strategy, Sinch

Email bounce rates and what they mean

Another reason emails fail to reach the inbox is because they’ve bounced. There are two main types of email bounces:

1. Hard bounces represent permanent delivery failures. They’re typically caused by an invalid recipient address or non-existent mailbox. Mailgun will automatically add contacts to your suppressions list when you receive a hard bounce.

2. Soft bounces represent temporary delivery failures. Reasons they occur can include a server outage, full mailbox, oversize files/messages, a blocklisting, or reputation issues.

So, hard bounces are more serious. They suggest you need to remove invalid emails from your list, fix authentication protocols, or mitigate a blocklisting issue. But how do you know what’s really going on?

We asked senders how well they understand hard bounces. Almost 24% of respondents said they were unsure about classifying hard bounces, 20% said they couldn’t classify them and 46% said they could.

We’re a little skeptical that 46% of senders truly understand how to classify hard bounces. While many email service providers (ESPs) show you hard and soft bounce rates, you might have to dig deeper to see the full picture.

When a message bounces, the receiving mail servers returns a specific email bounce code. These bounce classification codes represent different reason emails weren’t delivered.

Here are some examples of bounce codes:

• 5.1.1 indicates a hard bounce because of bad destination mailbox address such as an invalid email.

• 5.2.2 indicates a recipient’s mailbox exceeded its storage limit.

• 4.4.7 indicates a message took too long to be delivered and expired.

There are many others, including email bounce codes for SPF, DKIM, or DMARC failures. A hard bounce could also indicate a reputation issue with a provider like Gmail. Being able to classify hard bounces means you get specifics on what needs to be fixed. That’s why Sinch Mailgun offers bounce classification. It helps users focus on the critical bounces that need attention.

More than a quarter of senders say reducing and addressing bounces is a top email deliverability challenge. Understanding email bounces is vital to both maintaining a high delivery rate as well as list hygiene. The more you know, the easier it is to address high bounce rates.