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Why sending email isn't enough: How sender reputation impacts deliverability

Mailgun takes a deep dive into email reputation, and how numerous factors play a role in whether or not your email lands in the inbox.



When you think about the reputation of your business, customer service and product quality are key attributes that come to mind. Email communication practices can also contribute to the reputation of a business. For the purpose of this post, I’d like to deep dive into email reputation.

Poor email reputation directly impacts deliverability, slowing down your campaigns and sending them to spam folders. A good email reputation helps get more emails delivered to your recipients’ inboxes without delay. In addition, many of the techniques for maintaining and improving reputation will make emails more engaging, increasing open rates, click-through rates, and other measures of success all driving toward growth.

As the technical account manager for our deliverability service, I’m constantly working with large businesses and big brands on ways they can get more emails delivered to their intended recipients. Often times, email is the core driver of their growth and so if emails are not sent, growth is directly impacted.

The relationship between reputation and deliverability

Reputation consists of quantitative metrics tracked and used by inbox providers, such as Gmail, Yahoo Mail, and to rank senders (identified by IP and/or domain) by desirability. Each provider has a different algorithm for calculation so there is no single source for reputation.

It is very similar to a credit score: while every bank looks at the same information, they each have different ways of assessing credit-worthiness. To further complex the problem, those disparate algorithms are constantly evolving to stay ahead of clever spammers who find ways around them.

The following are core measures of reputation:

  • Third-party ratings and whitelists: Several companies provide independent ratings that inbox providers can use to determine a sender’s reputation. The best known and most popular of these is Return Path's Sender Score, which assigns a numerical rating from 0 to 100. A score of 70 or below typically indicates an issue with your email practices and can spark aggressive email filtering by inbox providers, but beware that each provider weighs these scores differently if at all.

  • Content flags: Over the years, providers have become more skilled at recognizing spam or junk. They monitor incoming messages for keywords and phrases and prevent what they determine is spam from getting to a recipient’s inbox.

  • Spam trap activity: To combat address scraping and other spamming practices, inbox providers keep a lookout for messages sent to email addresses known to be inactive or repurposed. They may even create user-less addresses that could not have opted into any list, these are known as pristine spam traps.

  • Lack of history: Much like credit scores, a lack of sender history can impact reputation. If a sender is using a new IP, many providers will limit access to recipients. The only way to improve your reputation is by warming up an IP, this is accomplished by sending emails that consistently engage recipients.

How reputation impacts deliverability

Reputation is a major part of deliverability, but exactly how one affects the other depends on the inbox provider. A message that arrives at Gmail inboxes might be identified as spam by Yahoo. Here are a few ways in which providers handle what they consider to be undesirable incoming email:

  • Throttling: This is the practice of limiting the number of emails an ISP accepts at one time, effectively causing a delay.

  • Spam designation: Labeling an incoming message as spam means that while it will reach recipients, it will fall into a spam or junk folder rather than the inbox.

  • Blacklisting: An ISP can outright reject messages from a suspect IP.

How to create and maintain a good reputation

1. Use best practices to grow and maintain email lists

  • Only send messages to users who have requested them - The best way to do that is by allowing users the opportunity to subscribe to email lists, for example on your blog or a landing page with helpful content. To be absolutely certain that a customer wants to subscribe, use a double opt-in process in which users are sent an email requesting that they confirm their interest before adding them to the list. This has the added benefit of confirming that the email address provided is correct and prevents bounce-backs, which can negatively affect reputation down the line.

  • Avoid purchasing email lists - It’s a tempting shortcut for marketers but they are never worth the trouble. Using an unfamiliar list can expose senders to spam traps, but that’s just the most overt damage they can cause. Senders are better off taking the slow-and-steady route of organic list growth and scrupulously maintaining their list hygiene.

  • Keeping email lists current is critical - In addition to responding to unsubscribes, complaints and bounces (see below), senders should monitor engagement levels by tracking opens and clicks. If a recipient becomes unresponsive, consider removing them proactively before they impact sender reputation. Exactly how you determine if these addresses are inactive depends on the types and rate of email you send, but six months of no opens for marketing emails is typical. Some senders also target these addresses for re-engagement campaigns, which asks them for feedback and gives them the opportunity to change their opt-in settings. In my experience, I have seen that inbox providers pay close attention to engagement so make sure this becomes part of your routine. Providers will even repurpose email addresses as spam traps after they have become inactive for a long period of time.

2. Get authenticated

Because spammers often pretend they’re someone they’re not, inbox providers take steps to ensure senders are accurately representing themselves. Senders should use all common forms of authentication to ensure they’ve covered their bases with providers by joining key verification registries, including:

  • Sender Policy Framework (SPF): This allows domain owners to publish a list of IP addresses that can send email on their behalf. SPF is simple to set up and makes it harder for spammers to spoof an email from a domain.

  • DomainKeys Identified Mail (DKIM): DKIM is an authentication method to prove an email originated at a specific domain and has not been changed during delivery. Senders that email on behalf of clients can configure their DKIM signature to use their domain so the “from” address domain and DKIM signature match when sending traffic over a dedicated IP.

  • Domain-based Message Authentication, Reporting and Conformance (DMARC): DMARC is a specification combining both SPF and DKIM to determine the authenticity of a message. Setting up a DMARC record can instantly improve your reputation as you are preventing unauthenticated parties from sending messages on your domain. DMARC records allow the sender to instruct email providers how to handle messages that don’t pass authentication. From what I’m seeing, DMARC is becoming more and more prevalent, so be sure to consider setting this up for all of your sending domains.

In a perfect world, reputation would be tied exclusively to a particular domain but reputation sticks to the IP addresses as well which means senders could inherit someone else’s poor reputation. To avoid earning a poor reputation based on another sender’s malicious or inappropriate activity, senders should maintain separate domains or subdomains for their marketing, transactional and corporate mail. This isolates the reputation of each type of email and assures that time-sensitive transactional and corporate mail won’t be delayed. Anyone sending more than 50,000 emails per week should use a dedicated IP to isolate their reputation.

Email service providers will likely throttle traffic from an unfamiliar IP address, so new senders should “warm-up” IPs gradually. This means sending emails at a low rate initially and then gradually increasing email volume as reputation improves.

When it comes to setting up DNS records, start with a well-regarded DNS provider and publish all contact information in the WHOIS record. Once senders have built an email reputation, they should consider signing up for whitelists. A whitelist is the opposite of a blacklist. It’s a list of pre-authorized email addresses that are much less likely to be blocked by spam filters.

3. Track and respond

One of the best ways to maintain email reputation is to keep an eye on what recipients are doing by monitoring bounces, complaints, and unsubscribe requests. Most large inbox providers provide feedback loops to high-volume senders. These are reports indicating how many recipients marker emails as spam. Some reports include specific addresses. After providers send the information, be sure to process hard bounces, spam designations and unsubscribe requests by removing recipients from the sender list.

Mailgun will automatically suppress these addresses, but it’s good practice to keep your lists up to date. Soft bounces, or undelivered messages due to temporary reasons, usually require additional analysis before a response. For one, many providers will soft bounce or throttle a certain number of initial emails from a new IP address. This is where warming up an IP is useful. For other soft bounces, such as full inboxes, senders can establish a designated number of attempts before permanent list removal.

4. Provide valuable content

As discussed previously, inbox providers monitor engagement, so senders should take steps to ensure their emails are attracting the attention of recipients. Senders should personalize their emails with content that reflects each recipient’s specific interests or usage patterns. Other keys to reader engagement, include:

  • Avoiding HTML-only email: ISPs don’t like it, so send multi-part emails using both text and HTML or text only.

  • Limit links: Having too many links and images can trigger spam flags so keep your text-to-link and text-to-image ratios as high as possible.

  • Use your words: Misspellings, promotional content (buy now!, Free!) are major spam flags, as are all caps and exclamation marks.

  • Include the sending domain in links: Using popular URL shorteners, such as Bitly, is a bad idea because they are frequently used by spammers.

  • Test and refine constantly: Sending different subject lines and content to portions of your list and measuring engagement (a process known as A-B testing) is an effective way to learn what content customers respond to best.

Conclusion — Be proactive

Creating and maintaining a good email sender reputation is much easier than trying to repair it later on. In addition, a good email reputation is more than simply a way to safely pass through the filters set up by inbox providers to protect their users.

When integrated into a comprehensive email delivery and monitoring policy, businesses can ensure they are reaching customers who want to hear from them and are fully engaged in their messaging. In turn, companies don’t waste money and valuable resources on email campaigns that fail to reach their targeted audience.

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