Email Marketing Deliverability and Spam

What is Email Deliverability

Email deliverability is about maximising the number of messages that arrive in the inbox rather than being blocked or marked as Spam by mailbox providers.

Maximising deliverability requires following a set of best practices demonstrating you are a responsible sender. The main referees are the mailbox providers, a.k.a. inbox service providers (ISPs), which include Gmail, Outlook, Yahoo Mail, AOL, and Apple Mail. If they do not trust you as a sender or think you are up to something suspicious, they may choose to send your message to the Spam folder or not deliver it at all.

Measuring email deliverability

Several reveal whether your email deliverability is in good or bad shape. These include:

  • Bounce rate
  • Delivery rate
  • Inbox placement rate

Bounce rate

An email bounces when not delivered, which could happen for several reasons. There are two different types of email bounces:

  • Hard bounces
  • Soft bounces

1.      Hard bounces

A hard bounce occurs when delivery permanently fails, indicating a serious email deliverability problem. Reasons for a hard bounce include:

  • Mailbox provider blocked the message
  • Fake email addresses
  • Typos in email addresses
  • Non-existent domain name

If you have a high number of hard bounces, it could lead to getting blocked by the mailbox providers. A few hard bounces are inevitable and likely related to invalid emails. However, you should remove those contacts from your list as soon as possible is important to protect your sender reputation.

2.      Soft bounces

Soft bounces indicate a temporary email deliverability problem. It is less serious than a hard bounce but is still something you should monitor. Common reasons for soft bounces include:

  • Full inbox
  • Email is too large to be delivered
  • Receiving email server is down/offline.
  • Mailbox configuration issues.

You can try resending to email addresses that result in soft bounces. But if it continues, those contacts should be removed from your list.

While the bounce rate varies depending on several factors, you will generally want to keep your bounce rate (including hard and soft bounces) below 1%.

Delivery rate

The delivery rate is the percentage of emails reaching recipients’ mailboxes. This includes all folders, including the inbox, Spam, promotions tab, etc., i.e. any email that didn’t bounce. You must aim for a delivery rate of 95% or higher.

Inbox placement rate

Another way to measure email deliverability is inbox placement, the percentage of messages that make it to the main inbox. It excludes anything quarantined to Spam or delivered to other folders instead.

Email deliverability issues

Spam traps

Mailbox providers seed fake emails around the web to catch potential spammers. If you are caught using one of these emails, it is a sure sign you are using illegitimate sources for your emails, e.g. scraping the web.

Spam complaints and unsubscribe rate

Mail providers allow their users to report emails as Spam. A complaint rate that is under 0.1% is considered acceptable.

Unfortunately, recipients frequently mark legitimate emails as Spam. They may just be bored of hearing from you as you have been emailing them too much for their liking and they cannot work out how to unsubscribe. For this reason, ensure that there is an easy way to let subscribers opt out of your emails.

Unsubscribes are a fact of life. However, a high unsubscribe rate can harm email deliverability. If mailbox providers see that many people are unsubscribing, they may take this as a signal that you are a spammer. Consequently, aim for an unsubscribe rate should be significantly less than 0.5%.

If your unsubscribe rate nears 0.5%, it is time to clean your list by removing inactive subscribers and work to increase email engagement with strong subject lines and compelling email content.

Authentication protocols

Email authentication is the technical setup part of email deliverability. It requires using DNS TXT records to be added to your sending domain servers, and specific information included in the email header. When a receiving mail server receives a new message, it uses the header information and and the DNS TXT records to verify the sender’s authenticity.

There are three records you need to set to maximise deliverability:

  • Sender Policy Framework (SPF)
  • DomainKeys Identified Mail (DKIM)
  • Domain-based Message Authentication, Reporting and Conformance (DMARC)

Your email platform should be able to guide you on how to set up these attributes for your sending domain.

SPF Record

An SPF record is a list of domains and IP addresses allowed to send mail on your behalf. When receiving mail servers want to authenticate a message, they compare the sending domain or IP with the list published on your DNS.


DKIM authentication involves encrypted keys that help verify a sender’s identity. When you set up DKIM, a public key is published in the DNS record, and a private/encrypted key is sent with the email. The private key is also a digital signature, which should be unique to the sender. DKIM makes it easy to prove the origin of the email and prevent spoofing.

The receiving mail server pairs the encrypted and public keys to authenticate the message. Email deliverability issues are more likely if the keys don’t match up.


DMARC tells the world that you want your email to be authenticated. This is how you let people know the email was sent from a legitimate source.

You must configure DKIM and SPF before configuring DMARC. DKIM and SPF should be authenticating messages for more than 48 hours before turning on DMARC.



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