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Understanding Undeliverable and Blocked Bounces

CTCT Employee

Here at Constant Contact, we are an industry leader in email delivery.  This is no accident.  All of our Terms and Conditions are designed to ensure the best possible delivery rate.  That being said, it’s expected that most email campaigns will have some level of bounces.  Knowing the cause and reasons behind these bounces can help you as a marketer to ensure you are mailing to the cleanest, most engaged list that you can.  Last month, I spoke about Non-Existent and Suspended Bounces, in this post I will be detailing Undeliverable and Blocked bounce types.


What Are Undeliverable Bounces?


When Constant Contact sends an email to a recipient, but the receiving server can not be found or connected to, the email will bounce as Undeliverable.


For Example: If someone were to send an email to my address, let's say it's; however, at that time the receiving server, in this case it would be example.comis not responding. The email will bounce as Undeliverable as a result (usually after multiple automated resend attempts).


Isn’t That The Same As A Non-Existent Bounce?


Not at all!


A Non-Existent bounce occurs when we connect to the receiving server, but they can not find the address we are trying to send to.


With Undeliverable bounces, that initial connection can’t even be made.


What Are Blocked Bounces?


In the never-ending battle against spam, ISPs do what they can to stay ahead of the curb.  Most often, this involves the use of email filters or blocklists to keep there subscribers’ inboxes clean.  Sometimes those filters and blocklists end up preventing Constant Contact’s mail from getting to the intended recipient, resulting in a Blocked bounce.


Does A Blocked Bounce Mean That Constant Contact Is Blocked?


Not necessarily,  While it’s true that Constant Contact (like all ESPs) may occasionally experience blocking issues at a particular domain/filter/blocklist, most often Blocked bounces occur on a smaller scale.  These bounces can occur due to content in the email that a spam filter deems problematic, such as an image, email address, or website.  Some ISPs even have rules based around how much mail they accept at a time from any one source, or even block email from bulk senders altogether.


Where Can I Find My Undeliverable and Blocked Bounces?


If you look at the reporting for a specific campaign, click on the number of bounces you have.  Then select “Undeliverable” or “Blocked” from the “Display” drop down.




What Should I Do With Undeliverable and Blocked Bounces?


In the case of Undeliverable bounces, it’s acceptable to try sending to the addresses again after a few hours, just in case it is a temporary issue.  If they continually bounce, then we recommend moving them to unsubscribe.


Blocked bounces sometimes take a bit of work to diagnose.  First, check the “Email Delivery” row at to see if there are any known issues.  If that doesn’t help, then please contact our Support Team to troubleshoot.


Hello, I'm Robb, your friendly neighborhood Compliance Representative. I've worked for the Constant Contact Compliance Team in some capacity since September 2008. I'm happy to use my experience to help educate and inform the Community on various Compliance and Delivery-related topics. In my spare time, I'm usually spending time with my wife, son and dog OR engaged in some sort of hockey/football discussion.