Email deliverability causes and solutions (part 1 of 2)

By Steve Kellogg

Today’s blog post “Email deliverability causes and solutions (part 1)” by our long-time friend and 10-year marketing automation veteran, Steve Kellogg, was originally posted on LinkedIn

We work hard to protect, nurture and grow our email DBs. For many, it’s our most effective marketing channel. Yet, according to Return Path, 20% of our emails never make it to the Inbox. Why? Only a small percentage of email deliverability issues are the result of bounces. The rest are being driven by 2 significant factors:

  1. Increased protection by ISPs against malware attacks and other malicious activities.
  2. Growing complaints by email recipients about unwanted/irrelevant emails, causing ISPs to implement their own mitigation strategies and solutions.

Below is a comprehensive summary on the subject, both from my own learnings and from other experts.

What affects email deliverability?

ISPs (such as Gmail, Hotmail, or Yahoo) decide which emails land in the inbox and which are blocked or bulked, all based on your overall email sender reputation.

  • Blocked: The ISP takes action against poor IP reputation and refuses to accept your emails altogether. It doesn’t get worse than this.
  • Bulked: The ISP accepts your email but routes the message to BULK or SPAM folders vs. Inbox. This is also determined by your overall sender reputation.

How is sender reputation measured?

Each ISP measures your IP/domain reputation separately based on several different metrics. Repeated issues with one or more of the red flags below will increase the chances that an ISP will block or bulk your messages.

  •  Bounces
  • Complaints
  • Engagement
  • Authentication
  • Volume and Frequency
  • Blacklists and Spam Traps


There are two main types of bounces: soft bounces (temporary failures) and hard bounces (permanent failures).

  • Soft bounces technically have made it to the email server, however they did not reach the sender. This can happen because:
  • You’re sending to someone who’s mailbox is full
  • You’re sending to someone who’s account has been temporarily suspended
  • There’s been an error or outage at the receiving mail server
  • There is a problem with authenticating you as a sender, or you have a high degree of other reputation issues
  • BTW, an out-of-office reply is technically not a soft bounce, as it did make it to the inbox.
  • Hard bounces happen when the domain does not exist, is invalid, or the mailbox has become inactive. It is a permanent failure.

According to Oracle deliverability expert Pradeep Mangalapalli, bounce rates up to 3% for hard bounces and up to 5% for soft bounces are considered acceptable thresholds.


ISPs track every time a subscriber flags an email as spam. In some cases, ISPs share this information with email senders through feedback loops (a process by which certain email senders may be able to receive notification when recipients report emails as spam).

There’s a lot more to sender reputation than a spam complaint; however, a recipient marking an email as spam is the strongest negative signal to ISPs about an email. Spam complaint rates above 0.2% are considered high, and may result in poor deliverability. At ISPs, like Gmail, a spam rate as low as .08% can “start to affect” your deliverability.


ISPs track how subscribers engage with your emails through opens, clicks, scrolling through a full email, deleting without opening and marking as read without opening.


Spammers will spoof legitimate domains to send their emails, so it’s important to adhere to all the latest security standards to protect your subscribers and your brand. ISPs will block messages that don’t pass these authentication protocols:

  • Sender Policy Framework (SPF): ISPs cross check the domain in your From address against the IP address listed in the public record in the Domain Name System (DNS). ISPs generally don’t block email solely because of a missing SPF record. However, it is one more data point that contributes to a sender’s reputation and it helps protect your brand.
  • Domain Keys Identified Mail (DKIM): Email senders generate public and private key pairs. ISPs lookup the public key in the public DNS record, and verify the matching private key in the email header. These days, messages not signed with a DKIM signature are very unlikely to see the inbox.
  • Domain-based Message Authentication, Reporting & Conformance (DMARC): The purpose of a DMARC record is to tell inbox providers WHAT you want them to do with email that doesn’t pass SPF and DKIM: allow it, filter it, or reject it. Soon, publishing a DMARC record will be necessary to ensure good deliverability to reputable inbox providers.

Volume and Frequency

When spammers spoof a domain, they take the opportunity to send extremely high volumes of email all at once. ISPs look at the history of your email volume and frequency. If there’s a spike, this can affect your reputation. Sending higher volumes of emails is okay as long as you are consistent. ISPs are more concerned with past trends and spikes rather than overall volume.

Blacklists and Spam Traps

Spam traps are email addresses that either belong to an ISP or belong to an inactive user. Often ISPs will monitor email addresses that have been inactive for a long period of time, and convert these dormant email addresses to spam traps.

There are two main types of spam traps:

  • Pristine: Email addresses that are created and published online but never register for any marketing emails. If a spammer crawls web pages to harvest published email addresses, they may pick up a pristine spam trap.
  • Recycled: If a mailbox goes inactive for a long time, the ISP will convert it to a spam trap to identify senders that mail to inactive subscribers. It’s not known for sure how long an email address remains inactive before it is converted, but anything over 12 months is generally suspect.

The net impact of a spam trap is that you get blacklisted. Landing on a blacklist can have various effects on your deliverability. In some cases it’s not a big deal, however getting on some blacklists, like SpamCop, can be severe and can take significant time to correct.

Email Setup

Emails that have HTML errors, such as image references or links that are dead can cause deliverability issues.

Stay tuned for “Email deliverability causes and solutions – part 2” next week where Steve discuss sender scores and best practices to mitigate deliverability issues.

Sources used for this post: Oracle Deliverability PlusLitmusReturn Path