June 6, 2022 Steve Kellogg

Email Deliverability Causes vs. Solutions- Part 1

We work hard to protect, nurture and grow our email databases. For many, it’s our most effective marketing channel. Yet, according to Deliverability experts, up to 20% of our emails never make it to the Inbox. What gives?

In this 3 part series we’ll discuss: 

  1. What affects email deliverability.
  2. A basic understanding of email settings and the deliverability impacts of each.
  3. Best Practices that will keep your email delivery healthy.

To kick off part one of our series we need to ask, why do 20% of our marketing emails never even make it to an inbox? You might think bounces are the main culprit, but only a small percentage of email deliverability issues are the result of bounces. The rest are being driven by two 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.

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. We’ve identified 6 red flags that will increase the chances that an ISP will block or bulk your messages. To start, we’re going to take a closer look at the first three to see how they impact deliverability.  


There are two main types of bounces: Soft bounces (temporary failures). 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 it to someone whose mailbox is full
    • You’re sending it to someone whose 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

It should be noted that an out-of-office reply is technically not a soft bounce, as it did make it to the inbox.

Soft bounces often end up resolving themselves, but since ISPs manage soft bounces differently, a check with your most prevalent ISPs on how many times (or how long) before they convert a soft bounce to a hard bounce is your best guide on when to take action on these.

Hard bounces happen when the domain does not exist, is invalid, or the mailbox has become inactive. It is a permanent failure.

The generally accepted industry standard bounce rate is 2%. Anything more than that should trigger a closer look. 5% or more means there is a serious problem and action needs to be taken immediately.


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 notifications when recipients report emails as spam).

There’s a lot more to a sender’s 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 interactions they have access to, including opens, clicks, scrolling through a full email, deleting without opening, and marking as read without opening. 

In our next Email Deliverability blog, we’ll dive into the other 3 red flags that we have identified and review which email settings can trigger ISPs to block your emails entirely. Stay tuned!