Overview

Email Delivery Rate is the ratio of the number of emails that were successfully accepted by the recipient server to the number of emails that were sent.

Reasons for Low Email Delivery Rate

The reasons for a low email delivery rate are as follows:

Hard Bounces

If an email address or domain doesn't exist, it is classified as a hard bounce. A high hard bounce rate indicates that your email list quality is bad and many of your users don't have a valid email address. Hard bounces may also occur due to the following reasons:

• Unsecure audience collection
• Lack of overall list hygiene.

In MoEngage, all users marked as hard bounces are suppressed from all future mailings. 

Soft Bounces 

These occur due to temporary failures such as a full mailbox, a recipient’s domain being unavailable or unreachable, IP or domain blocks, emails being blocked due to content getting flagged as spam, or blocks due to the low reputation of the domain/IP. Soft bounces indicate a temporary issue with email delivery, which can be resolved later. High soft bounces result from bad list quality and/or bad sending practices.
In MoEngage, users marked as soft bounces are not suppressed from future mailings. 

Deferrals

This is a transient state where an email was sent, that is, a delivery attempt was made, but it was not delivered or failed. The recipient server responded that the server was busy or the IP was rate limited/throttled and retried later. So, the MoEngage system retries these emails until they are delivered or failed (bounce).

Email deferral is not the final state of the email. It gets delivered or bounced with subsequent reattempts until the pre-configured bounce-after period, typically 72 hours for Sendgrid and 14 hours for Amazon SES. So, after 72 hours of the first delivery attempt, if the email is still in the queue, it will be marked as a soft bounce.

Deferral is the intermittent event between the sent and the outcome of that sent, which can be delivered or bounced. Also, one sent event can have multiple deferrals. In contrast, one sent event can only have one of the delivered or bounced events.

Lateral Bounces 

These are exceptional cases where the recipient servers initially accept the emails (delivered) and then reject them 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

A high hard bounce rate indicates that your email list is old and/or is not getting cleaned regularly. Perform the following steps to reduce hard bounces:

•  Verify that the auto-suppression mechanism for hard bounces works as expected.
• Check the audience collection points. For new customers, secure your 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 old unengaged users regardless of whether the email address is valid after some inactivity (say 9 months).
• Clean the database using a third-party service if you haven't sent emails in a long time or have offline signups.
• Send a welcome campaign to new users.
• Abstain from purchasing lists.

Reduce Soft Bounce

Perform the following steps to reduce soft bounces:

• Make sure hard bounces are low.
• Ensure domain and IP reputation is either medium or high.
• Check for IP blocks and apply for remediation. Suppress users from those domains until the blocks are lifted.
• Suppress users who continuously soft bounce due to the mailbox being unavailable or full.
• Test the email content (HTML) and make sure that it is getting filtered out by spam filters.

Reduce Deferrals

• Warm up and Ramp up emails correctly for new senders.
• Existing senders should refrain from causing sudden volume spikes.
• Use low Request Per Minute (RPM) to throttle all one-time campaigns.
• Use Best time to send (BTS) for higher volume campaigns, especially if you send high volumes infrequently.
• Ensure the number of IPs is adequate to support the volumes, especially for regional domains.
• If you see any issues, pause and send a smaller volume to previous email openers, and then scale up properly before the issue escalates.