July 7, 2022 Steve Kellogg

Email Deliverability Causes vs. Solutions- Part 2

In our last Email Deliverability blog, we looked at what affects email deliverability which included bounces, complaints, and engagement. In this blog, we’ll review which email settings can trigger ISPs to block your emails entirely. First up, authentication.


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 the following 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 an 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 look up 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 emails 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.

Blocklists and Spam Traps

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

Let’s take a closer look at the 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 blocklisted. Landing on a blocklist can have various effects on your deliverability. In some cases it’s not a big deal, however getting on someone’s blocklist, 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.

Now that you know what to look out for, what’s next? In our final blog on email deliverability, we’ll provide Best Practices that will keep your email delivery healthy and happy.