Overview

Email delivery rate is the ratio of the number of emails successfully accepted by the recipient server to the number of emails sent.

Reasons for Low Email Delivery Rate

The following are factors that contribute to a low email delivery rate:

Hard Bounce

A hard bounce is a permanent failure and occurs when the recipient's email address or domain doesn't exist. A high hard bounce rate indicates that your email list quality is bad and many recipients don't have a valid email address. Hard bounce may also occur due to the following reasons:

• Unauthorized audience collection: This refers to the practice of collecting email addresses and building an email list using methods that lack proper permission or consent from the recipients.
• Lack of email list hygiene: This refers to the fact that the email list is not regularly maintained and may contain outdated or invalid email addresses.

Soft Bounce

A soft bounce is a temporary failure and indicates a temporary issue with email delivery, which can be resolved later. A high soft bounce rate indicates bad email list quality and/or bad email delivery practices. Soft bounce may also occur due to the following reasons:

• A full mailbox
• A recipient’s domain being unavailable or unreachable
• IP or domain blocks (emails are blocked due to content getting flagged as spam)
• Blocks due to the low reputation of the domain or IP

Deferral

An email deferral is when an email server temporarily delays an email from being delivered. This is different from a dropped email, which is when an email server permanently refuses to deliver an email. Deferral can be considered a transient state when an email is sent but cannot be delivered successfully or encounters a failure, indicating a temporary state of pending delivery.

Deferrals may occur when the recipient server responds with messages such as

• Server busy
• IP rate limiting or throttling

This prompts MoEngage to retry sending these emails until they are successfully delivered or marked as failed (bounced). Once deferred, the email undergoes subsequent reattempts until the pre-configured bounce-after period, after which it gets successfully delivered or marked as a bounce. The bounce-after period is typically 72 hours for Sendgrid and 14 hours for Amazon SES. Therefore, if an email remains in the queue after 72 hours of the initial delivery attempt, it is classified as a soft bounce.

Lateral Bounce

Lateral bounce is an exceptional case in which the recipient servers initially accept and reject the emails later. This usually occurs due to a misconfiguration in the recipient servers' incoming mail handling settings.

Improve Delivery Rates

Hard and soft bounces are the primary reasons behind low delivery rates; decreasing their number helps improve your delivery rates.

Reduce Hard Bounce

Perform the following steps to reduce hard bounces:

• Verify the auto-suppression mechanism: An auto-suppression mechanism is a system or process implemented by email service providers (ESP) to suppress or stop automatically sending emails to bounced recipients. Verify that the auto-suppression mechanism for hard bounces works as expected.
• Check the email list collection points: For new customers, secure your email list collection points to ensure that valid email addresses are collected. You can use double opt-ins or captchas in the sign-up form or ask the customer to re-enter the email address to avoid typos.
• Remove unengaged users: After some inactivity (approximately 9 months), remove these users regardless of whether their email addresses are valid.
• Clean the database: Cleaning the database using a third-party service is effective if you haven't sent emails in a long time or have offline signups.
• Send a welcome campaign to new users: A well-executed welcome campaign helps establish a strong foundation with new subscribers, builds trust, verifies email addresses, encourages engagement, and improves list hygiene. These factors collectively contribute to reducing hard bounces.
• Abstain from purchasing lists: Purchased lists often contain outdated, invalid, or non-existent email addresses. By abstaining from purchasing lists and instead using authentic permission-based methods to build your email list, you can ensure that your email addresses are current and valid. This significantly reduces the likelihood of encountering hard bounces.

Reduce Soft Bounce

Perform the following steps to reduce soft bounces:

• Ensure hard bounces are low: While hard and soft bounces are independent, managing hard bounces effectively contributes to maintaining a healthy email delivery infrastructure. Minimizing permanent delivery failures creates better conditions for successful email delivery and thus reduces the likelihood of soft bounces.
• Ensure your domain and IP reputation is medium or high: By establishing and maintaining a favorable domain and IP reputation, you create an environment that fosters successful email delivery, increases recipient trust, and reduces the likelihood of soft bounces. For tips to raise your domain reputation, refer to Domain Reputation Issues and Preventive Measures.
• Check for any IP blocks: If IP blocks are identified, take appropriate action to resolve the issue. While the IP blocks are still active, it is recommended that recipients from the affected domains be suppressed until the blocks are lifted.
• Suppress recipients: You can optimize your email delivery by suppressing recipients who consistently soft bounce due to mailbox unavailability or fullness. This helps you target active and responsive recipients, improving engagement rates and reducing soft bounces.
• Test the email content (HTML): Thoroughly testing your email content and ensuring that it avoids triggering spam filters can reduce the chances of your emails getting bounced or being marked as spam. This leads to improved deliverability, increased engagement, and reduced soft bounces.

Reduce Deferrals

Perform the following steps to reduce deferrals:

• Warm-up and Ramp up emails correctly for new senders: This is a proactive approach to establishing a good reputation with email providers. It reduces the chances of deferrals by gradually increasing the email volume and demonstrating responsible sending practices, ultimately improving deliverability and email campaign success.
• Avoid causing sudden volume spikes: When an email server receives an unusually high volume of messages within a short period, it may become temporarily overloaded or experience resource limitations. In such cases, the server may respond by deferring the delivery of some emails. Therefore, existing senders should refrain from causing sudden volume spikes.
• Use low request per minute (RPM) to throttle one-time campaigns: Implementing a low RPM limits the number of email requests per minute. This allows the server to handle the requests more effectively and reduces the likelihood of deferrals. For best practices, refer here.
• Use the Best time to send (BTS) strategy for higher volume campaigns: Using the BTS strategy is helpful, especially when sending high volumes infrequently. This helps optimize email delivery by increasing the chances of timely and successful email delivery, thus reducing the potential for deferrals.
• Ensure that the number of IPs is adequate: The adequate number of IPs supporting the email volumes, especially for regional domains, can help reduce deferrals in email delivery.
• Pause and send smaller volume: Pausing and sending a smaller volume to previous email openers, followed by a proper ramp-up process, helps reduce deferrals in email delivery.