High Bounce Rate


If you've been directed to this article, Mandrill noticed that you may have a higher than average bounce rate and need help with transactional email best practices. It's important that you read through this guide to help improve your account reputation and sending quota, and to ensure the emails you send through Mandrill conform with the Mandrill Terms of Use.

Bounces are undeliverable emails, typically because the receiving server or mailbox encountered some sort of delivery issue.

Mandrill uses a variety of factors to set an account's reputation and hourly quota. One of these factors is the number of hard and soft bounces relative to your overall sending volume.

After you remove bounced emails from your database and take steps toward preventing bounces, your account reputation and quota will adjust automatically based on the new sending patterns.

Target Bounce Rates

While bounces are considered normal when sending email over a period of time, industry standards generally suggest that your overall hard bounce rate should be below 5% of your total email volume. At this level, some receiving servers may start blocking delivery of your mail or decrease how quickly they are willing to accept your mail resulting in delivery delays.

Like most ESPs, Mandrill actively manages the IP addresses that are used for sending mail through our system. We need to be able to send mail quickly, with as little delay from the receiving servers as possible. The reputation of our IP addresses is paramount to ensuring high deliverability for all of our users.

Bounce rate thresholds fluctuate over time based on factors like the incoming volume to a particular ISP. Because of this, and to avoid providing insight that could assist spammers in gaming their system, ISPs don't publicly release their thresholds and schedules. We recommend that you aim for a rate under 5%, but understand that this is just a rule of thumb and can vary by ISP and other factors.

Reasons for High Bounce Rates

  • Old or stale recipient email addresses. Email addresses have a short lifespan. Recipients change jobs, change names, and create and delete email addresses at will. As a result, email address can become invalid over time, so only send to clean and current addresses to avoid issues with stale lists.
  • Fake email addresses or email addresses that don't exist are also a source of bounces.
  • Using unconfirmed or single opt-in lists. If recipients aren't required to confirm their subscription, this opens the door for allowing typos, old, fake, or stale email addresses into your list.

Reducing or Preventing Bounces

Remove stale or old email addresses. If you've been sending on a regular basis and tracking opens and clicks, you may consider removing inactive recipients. Inactive recipients are people who haven't opened or clicked your emails in several months.

This will help eliminate bounces and ensure a high-quality list and high engagement. High engagement helps your deliverability as many ISPs use engagement-based filtering. You'll also ensure you're only sending content to people who want it and who are more likely to engage with you and your company.

Clean your list prior to sending. Mandrill should not be used to clean your list, and there are a number of online services that can do this for you.

Validate email addresses before adding to your application or database. If users are inputting email addresses through an online form, validate the email address using a bit of code to ensure the email is technically sound (i.e. contains only one @ symbol, has a valid local and domain portion, etc.). You can also require that users type the email address twice and make sure that both instances match.

Be careful with online contests or promotions. For example, if you run an online contest and require that users enter in their own email address as well as refer a friend, they may be inclined to enter a fake or non-existent email address for the 'friend', to bypass that requirement. The same is true if there’s no verification of their email address before offering an incentive or reward—you're then left with emails in your database that will eventually bounce.

Use a confirmed or double opt-in process when collecting email addresses. When you use the double opt-in method, you're targeting only those recipients who really want to hear from you. Double opted-in lists have much higher engagement levels over time which translates to more opens and clicks and fewer bounces and unsubscribes. When you use a double opt-in process, people won't be able to sign up with fake, non-existent, or stale email addresses. They have to have a valid email address to opt-in.