Monitoring Email Deliverability: Bounces - Part I

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Having a clean list is essential to maintaining good email deliverability. Marketo does some, but not all, of the work for you. This blog will be the first in a multi-part series on monitoring your email deliverability.

First up, identifying unsuccessful email addresses. Many people think that Marketo stops emailing anyone who bounces but that is not true. We stop emailing anyone who bounces because of a bad email address, but that’s not the only reason a mail server would give us a bounce. Think of a hard bounce as the recipient going “nope, never going to receive that email, no matter how many times you ask me” and a soft bounce as “well, maybe later but not right now”. Bounces fall into three main categories:

  • Incorrect email address
  • Anti-spam block
  • Technical issues
  • Soft bounces

Let’s take a quick look at what happens in Marketo for each of these categories.

Incorrect email address

If the email address does not exist, it is a Hard Bounce and the email will be marked as invalid. The Lead record contains a field called “Email Invalid” which will be set to “true”. Also, the field “Email Invalid Reason” will contains the reason, if provided by the receiving mail server. This is the only type of Hard Bounce for which the Email Invalid field will be set to “true”.

The easiest way to see a list of this type of Hard Bounces is to look at the System Smart List “Bounced Email Addresses”. You can also create a Smart List with the filter “Email Invalid = true” and add a filter “Email Bounced” to specify the email for which it bounced (optional). Make sure you add the column “Email Invalid Cause”, so you can analyze the bounce reason.

Anti-spam block

If the receiving email server classifies your email as spam, it will also give a Hard Bounce. Marketo will stop trying to deliver this message and it will suspend all emails to that email address for 24 hours. This is done to protect your email reputation. After 24 hours, you can again send email to this email address. Marketo will record the cause of the suspend  in the field “Email Suspended Cause” and the suspend date in the field “Email Suspended At”. The field Email Suspended will be set to True temporarily.

To see a list of Blocks, create a Smart List that contains “Email Suspended Cause = not empty” and add a filter “Email Bounced” to specify the email for which it bounced (optional). Make sure you add the columns “Email Suspended Cause” and “Email Suspended At”, so you can analyze the bounce reasons.

Technical issues

If Marketo receives another error code or does not receive a response at all, there is usually a technical issue somewhere along the path from sender to recipient. This is classified as a Technical Bounce. Because this error is not caused by an anti-spam block or a bad email address, it is possible to immediately send other emails to this same email address. Also, neither “Email Suspended” nor “Email Invalid” is checked. This type of error is usually only a small percentage of an email campaign and it’s not something that you can take action on, because it’s a third party technical error. Usually the issue is resolved by the next time the lead is scheduled to receive email.

To see a list of Technical Bounces, create a Smart List with filters “Email Suspended Cause = empty”, “Email Invalid = FALSE” and a third filter “Email Bounced” in which you specify the email.

Soft bounces

When the receiving email server can temporarily not deliver a message, it tells Marketo to try again later. Marketo will try to redeliver the message several times for a period of time.  During that time it is shown as Pending in the Email Performance Report. When the message is still not accepted after 24-36 hours, Marketo will stop trying and report the message as a Soft Bounce. Since there is nothing you – as a sender – can do about temporary issues on the side of the receiving email server, there is nothing you can do to reduce the number soft bounces.

If you want to see a list of email addresses that soft-bounced, create a Smart List with the filter “Email Bounced Soft” and specify the email it bounced from (or choose “any” in case you want to see all bounces).

What should I do about them?

If we’re seeing repeated bounces due to technical or spam reasons to the same email address, you may want to mark these leads as Email Invalid and stop emailing them. If there are recurring failures over time, it is likely that email will never deliver to those addresses. Mailing to these can be risky because domains that have expired are sometimes turned into spam traps.

Here’s an example of a smart list that you could use to identify email addresses that are bouncing repeatedly (note the advanced filters):


(If you are emailing at least once a month, we suggest looking at a three month timeframe.)

On my team, we set up campaigns to marketing suspend leads that repeatedly bounce, with a custom reason field to keep track of whether it was because of hard or soft bounces. It is important to be proactive about dealing with these records, particularly when you are on a shared IP address with other clients. I personally like to set and forget on this kind of data management campaign, but you could also take a more proactive approach and review your chronically bouncing email addresses once a month and deal with them differently based on the reasons/frequency of the bounces. Just as long as you’re doing something about it!

Level 9 - Champion Alumni

I tried to rate this article, but was unable to do so.

In the' Incorrect email address' section, you state "If the email address does not exist, it is a Hard Bounce and the email will be marked as invalid."  That is true if it generates a Category 2 Email Bounce, but there are other Hard Bounces that can indicate an email address does not exist and Marketo does not automatically change the Email Invalid field value to 'true' for these.  I created a smart campaign that changes the Email Invalid field value to 'true' for Email Bounces that contain the following in their Details field.

  • invalid recipient
  • recipient invalid
  • recipient not
  • recipient unknown
  • unknown recipient
  • recipient rejected
  • Recipient address rejected
  • User address
  • user invalid
  • user not
  • invalid user
  • user unknown
  • unknown user
  • invalid address
  • Unknown address
  • Unable to verify destination address
  • Mailbox unavailable
  • no such
  • no longer
  • Non-existent
  • Bad email
  • email bad
Not applicable

Thanks Elliott, that's a great addition!

Level 4

Thanks Elliott--I am going to do the same with creating a smart campaign. Very helpful article.

Level 2

Thank you Kristen and Elliott for your contributions. A few comments/questions:

1) We've seen people 'hard bounce' and then be delivered on the same day. I don't understand how that fits into “nope, never going to receive that email, no matter how many times you ask me”

2) I set up Elliott's smart campaign for testing in our instance and it initially looked like a handful of people were identified and I was happy. However, I noticed that several of the people had accumulated lead scores. I adjusted the smart campaign to be people 'not delivered an email' in the past 7 days and it shrank down to 2 qualifying leads. So some people fit the criteria above but were also currently being delivered email.

Level 9 - Champion Alumni

Keep in mind that an email can have both an Email Delivered and an Email Bounced activity, so excluding only on the basis of Email Delivered, can result in incorrect results.  You can read more in the Under what circumstances can an email be delivered and bounced?  discussion.

Level 7

Marketo categorizes bounces a little differently than the rest of the industry.  The different treatment of Hard Bounces based on the category of invalid address or spam block described in the article above is somewhat unique.  This can explain why you may see an email address Hard Bounce in one campaign and deliver in a later campaign.   The email may have Hard Bounced due to a spam block and then delivered successful later.

If you do ever see evidence that an email has bounced with an invalid address reason but is not set to email=invalid please escalate to Support.  Support will alert the Deliverability Team to make the necessary changes to our bounce categorization.

Thank you!

Level 9

Are you simply using, "Member of Smart List" = False, when sending emails to make sure these aren't being sent to?

Level 9 - Champion Alumni

Chris Saporito I believe that is what Kristen is saying.  However, keep in mind that you can have both a Email Delivered and Email Bounced activity, so you may want to omit the Email Delivered filter from the smart list.

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An excellent Marketo integration for email deliverability is StrikeIron. It can take any new leads that are created and check to see if the email domain exists, then checks to see if the person at that domain exists. You can also run your database through it once as a baseline clean and then set up an ongoing maintenance routine that rechecks each lead at a set interval. With their assistance and the items that Elliott describes, our email deliverability is constantly above 97%.

Not applicable

"I created a smart campaign that changes the Email Invalid field value to 'true' for Email Bounces that contain the following in their Details field."

- what is the details field?