8 Deliverability Factors You Must Know

Getting email delivered to the inbox requires doing a lot of things correctly. It is a fact of email marketing that you must follow certain guidelines to ensure ISPs (Hotmail, Gmail, Yahoo, etc) accept your email messages.


There are three main areas of deliverability that need constant attention and focus to ensure that your email is delivered to the inbox.

  1. Set-up procedures: authentication, CAN-SPAM
  2. On-going: bounce management, data hygiene
  3. Reactive: unblock requests, blacklists

I will discuss in this post what I think are the top 8 deliverability factors you need to know before sending your first email.


NOTE: I will not discuss one of the most important which is sending engaging email. Check out other posts on the blog for engagement strategies.

Many of the 8 factors should be managed by your Email Service Provider (ESP); however it is very important that you have some knowledge of what affects the deliverability of your email marketing messages.

Here are the top 8 deliverability factors you must know before sending your first email. This is a long article; so to make it easier to digest you can click on the title below and jump down to each one if you prefer.

  1. World-Wide Can-Spam Laws
  2. What is a bounce
  3. What Is A Spam Complaint?
  4. How To Monitor Your IP Reputation
  5. Know Threshold Metrics For Bounce, Spam, And Unsubs With The Major ISPs
  6. How to whitelist and apply for feedback loops
  7. Authentication requirements
  8. How to request a unblock

1. World-Wide Can-Spam Laws

There are legal aspects of sending email marketing messages that all senders have to abide by. These are global and each country has their own set of laws.

At a top level here is what you MUST do:

  • Don't use fraudulent transmission data, such as open relays and false headers
  • Don't use misleading sender or subject lines
  • Add your postal address to all email
  • Include a "clear and conspicuous" unsubscribe mechanism in every email
  • Have a process for handling unsubscribes within the 10-day window. Ensure this is in place electronically, as well as for unsubscribes received via postal mail
  • Offer recipients a way to receive some types of email from you while blocking others, along with a "global unsubscribe" option to stop all future email from your organisation

Here is further resource for your reading:



2. What Is A Bounce?

A bounce is when an email is returned to the sender because it is not accepted by the ISP. There are two different types of bounce:


Hard bounce
A hard bounce is a bounce returned to the sender because it is permanently undeliverable. Types of Hard bounce include:

  • Invalid user: an email address that does not exist anymore
  • Block: indicates that the recipient’s email server is blocking email from our email server

Soft bounce

A soft bounce is a bounce that is returned temporarily and can be retried. Types of Soft bounce include:

  • Inbox full (less common now because of inbox sizes increased)
  • Host not responding: the recipient’s email host is not responding
  • This could be due to a temporary issue with their network

3. What Is A Spam Complaint?

A spam complaint is when a recipient hits the junk or spam button on the email in their inbox. ISPs will then use this as a determination whether to direct further email into the junk folder or block it altogether. Therefore keeping the spam complaint rate down is very important. Unfortunately, some recipients choose to use the spam button as a secondary method of unsubscribing.


Spam complaint rate should be less that 0.1%.


ISPs, via feedback loops, let the sender know when the spam complaint occurs and who has complained, so they can be unsubscribed from your list. The following ISPs allow feedback loops and shown below is how to apply for them.

NOTE 1: Your ESP should have your sending IP on all of these Feedback loops before you start sending email.

NOTE 2: Gmail is not on this list. Gmail is the only ISP that does not allow feedback loops.

Earthlink: (email only): fblrequest at
Outblaze ( email ONLY: postmaster at
Rackspace (formerly Mailtrust):
RoadRunner/Time Warner Cable:
Unite Online/Juno/Netzero:
Yahoo! (requires DomainKeys or DKIM)


4. How To Monitor Your IP Reputation

Your IP reputation is a strong factor in what ISPs use to determine whether to accept your email and/or put it into the junk folder vs. the inbox.

IP reputation is an accumulation of many factors such as: bounces, complaints, volume, authentication, etc. The best way to monitor your reputation is to use Sender Score (

Sender Score is a free email reputation service provided by Return Path. It is a great way to get an IP reputation report for both your sending IP and sending domain.

The score provided by Sender Score is….

“Compiled through our cooperative reputation network, provider access to data that ISPs and other email receivers can use to determine where to accept or reject email”

Not all email networks or ISPs use Sender Score to determine whether they will deliver your email. However, the factors used in determining your Sender Score are similar to those used by email networks and ISPs to determine your sender reputation.


Go to and enter your IP address and you will see your score. Create an account and you will have access to the following further information:

The score…and information
The actual ‘score’ should be used as an indicator of your IP reputation. It is a calculation of the reputation measures (below) and is out of a 100. So 100 is good; 1 is bad.

Other factors:

Host name:
I’m using Crate and Barrel for this example. This is the sending domain associated with the IP address.
This will show if and what blacklists you may have been listed on. If you have a blacklisting recorded here, contact your ESP for a detailed explanation.

Return Path Certified and Return Path Safe:
These are associated with Return Path’s Email Certification Program:

The deliverability report and accepted rate shows the likelihood of email being accepted and delivered. It uses the Return Path network to determine the rate. This is a great indicator of your actual deliverability rate, and should match fairly closely with your email metrics.
Anything below 95% is a concern and action should be taken.

Reputation measures
The reputation measures area is my main focus as it gives you the most insightful scores.

A complaint is when a recipient of your email hits the spam or junk button. Complaint rate and avoiding complaints is a major factor in IP reputation. Anyone can complain: even if they have legitimately signed up for your email, they may still complain. ISPs use complaints to determine that your message was not wanted by the recipient.

Remember 1 is bad; 100 is good.

If your complaint rate is below 70% you should take notice. How do you reduce the complaint rate?

Volume is how much email you send. I have noticed that the more good email you send, the better your volume score and Sender Score. I also recommend avoiding spikes in volume.

Unknown users:
Unknown users are invalid email addresses, i.e email addresses that do not exist anymore. A low unknown user score is an indicator of bad list practices. These include:

  • Sending email messages to old email addresses that have no previous mailing history
  • Not managing bounces correctly - not removing invalid email addresses after the first bounce
  • Sending to rented or purchased lists – we all know not to do that though, right?

Spam trap hits and last spam trap date:
A spam trap is a seemingly valid email address used to identify spam messages. This includes using an email address (even an entire domain) that hasn’t subscribed to any email and monitor the email it receives. It can also include taking an old email address that was once subscribed and see what email it receives.


A spam trap hit is another indicator of bad lists practices, especially emailing old data. I recommend not emailing addresses that are over one year old (from signup).

While a spam trap hit is not a major reputation issue, it is still use as a reputation indicator by Sender Score and ISPs.


5. Know Threshold Metrics For Bounce, Spam, And Unsubs With The Major ISPs


When reporting on email marketing and deliverability there are some key metrics that you should monitor closely. These are indicators of how well your email is performing as well as how your deliverability is performing and your risk of email not going into the inbox. These metrics should be monitored on a mailing per mailing basis and also be compared month over month.

Bounce rate less than 5%

A bounce rate of over 5% is an indicator of bad data-hygiene. Are you sending to old email addresses? Have you purchased a list (I hope not).

Unsubscribe rate less that 1%

A unsubscribe rate of above 1% is an indicator of lack of relevancy and the recipients don't want your email anymore. Are you sending the same content to every recipient? Are you being relevant and timely?

Spam complaint rate less that 0.1%.

A spam complaint rate is an indicator that you are continually sending to inactive recipients and your email is not relevant any more. Do you send to email addresses that have not opened in the last 6months? Do you send the same content to all your recipients?

Have an above normal complain rate? See How to keep your complaint rate low (blog post link)

If your metrics are out of these thresholds then you need to address the underlying issue.


6. How to whitelist and apply for feedback loops

Some of the major ISPs allow you to whitelist your sending IP address if you meet certain criteria. Meeting the criteria shows you are good sender and therefore it is good practice to do.

A whitelist is a list of IP addresses that have proven to be used for permission-based email only. The ISP lets emails from that IP through, and they are much less likely to be blocked by spam filters. However it is not a guarantee of inbox delivery.

Your ESP is responsible for whitelisting the IP address you use; however it is your responsibility as the sender to follow best practices and ensure you stay whitelisted.


7. Authentication Requirements

When sending email messages you have to ensure you have the correct authentication in place. The authentication is there to prove you are a legitimate sender and not a malicious one. It is something that should be done from day 1 before you send any email.

SPF Records
SPF, or sender policy framework, is authentication that when you send email, your recipient’s mail server will evaluate your sending IP address in the public Domain Name System (DNS) to make sure it is allowed to send the email on behalf of the sender. AOL, Gmail, and many others are known to use SPF when checking authentication. 

Sender ID
Similar to SPF, Sender ID verifies the email sender’s IP address against the sending domain. Sender ID uses an algorithm, called Purported Responsible Address (PRA), to examine the message content of email by specifically focusing on your From Address.

Sender ID was developed by Microsoft and other industry partners and has been adopted by more than 10 million domains worldwide. MSN Hotmail is known to use Sender ID when checking authentication.


With DKIM, two corresponding keys are created. One that is public, stored in the DNS as text and a private key that is accessible just to the email server. Every time an email campaign is sent, a private key is included in the email message headers. When ISPs receive the email message in their servers, they then verify the public and private headers to ensure verification.

The keys verify that the sender and the email message were not changed in transit. Yahoo, AOL and Gmail are known to use DKIM when checking authentication. In addition, Hotmail also uses DKIM, but typically only when Sender ID fails. Find our more:


8. How To Request A Unblock

Unfortunately, it is inevitable that you will get blocked by one of the major ISPs. Don’t panic – there are clear ways to get ‘unblocked’. A block normally occurs if you have gone over the ISPs thresholds (as discussed above). When you do get a block you will see 500 error messages in your SMTP transaction. Most ISPs will tell you what the 500 error means.


You can submit a unblock request with the major ISPs here:

Comcast: Send email to following email address and include your private IP address in the body of the email:

There you have it; 8 Deliverability factors you must know before sending your first email.


Any comments or questions? Let the team know.

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