Sudden Increase in Email Campaign Bounce Rates?

  • 17 March 2024
  • 6 replies

Badge +1

I have noticed that recently my bounce rates have increased significantly from where they were previously. It seems as if the majoriy of them are coming from the domain. I have also seen hotmail/outlook/office365 and the second highest that much less than Comcast and some from something called “Proofpoint”. 

I’m not doing any differently with my email acquisition with emails subscribtions coming from people who actually touch my site and I’m using the same segments to exclude email addresses who havent engaged with any emails after a period of time.

I’m wondering if this could be do to a setting possibly from when I setup the new branded sender domain or if there is something else I may need to look into. 

I’m getting a litte concerned because though the rest of my deliverability metrics seems to be favorable this one clearly needs to be addressed.

Any advice on how to properly troubleshoot or correct this b/c I’m getting worried. Last thing I want is to be considered a bad sender.


Best answer by StefanUE 18 March 2024, 08:20

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6 replies

Userlevel 2
Badge +5

Hey @kaniayubu 

I hope you are well,

If you are seeing a pattern of these emails that constantly bounce, you can just create a segment of these profiles and exclude that segment from your campaigns moving on. The segment should look like this -

The segment will also show you approx number of profiles that match the condition, so you can also know the number of profiles in your account that have those domain emails.

If you already have a large number of these emails, you can export the segment as csv and permanently suppress them as well, this will help you clean your lists as well.

You can do the same for flows as well - You can exclude them from entering your flows - you can add a flow filter like - email ‘doesn’t contain’ - that would allow only profiles that doesn’t have in their email to enter the flow.

Apart from this, Klaviyo has some documentation and steps as well on how to reduce bounce rates in general

I hope this helps!

Arpit Banjara

Userlevel 5
Badge +18

@kaniayubu hey, thanks for your question!

To add a bit of context to what @ArpitBanjara said, issues with landline email providers (e.g. Comcast (Xfinity), Telstra...) are pretty common and they stem from very tight anti-spam filters they have in place to protect their users. What’s even more problematic is the fact that they often sweep their databases for inactive accounts and disable or remove them entirely. 

So, if you have a lot of users from these providers, it would be a great idea to apply Arpit’s solutions to exclude them if they have soft bounced, even if it’s just once.

Proofpoint is a very famous global anti-spam protection platform which would’ve probably rejected your email after it failed their spam filter, which shows up as a bounce in Klaviyo.

Hotmail/Outlook bounces are nothing to be concerned about and they happen all the time, so that’s to be expected. We’d normally exclude those from our regular sends and then send specific emails to this audience, excluding more and more different subgroups from it, until we’ve ascertained where the problematic part of the list is, which would then get suppressed. 

Key step for you to protect your deliverability is to immediately exclude all problematic domains from your sending (incl flows and campaigns both), and to then slowly troubleshoot by grouping smaller batches together and sending to them until your bad metrics go over the threshold. 



Just chiming in that this is exactly what is happening to us as well. We are getting countless customer service emails about it, and after segmenting I am back up to about 350 (and counting) whose emails are being bounced, some once again. Most or all from Comcast. Suppressing or deleting these subscribers would be a huge issue, as many of them are daily readers and shoppers whose emails continue to bounce, then they are suppressed. I’ve encouraged some to change their email address altogether, but that doesn’t seem like an ideal solution for the customer. 

All is to say, I am following along here for advice because this is a constant problem now that I haven’t ever seen before when we had Mailchimp, and wondering why it is such an issue now. I’ve also been constantly cleaning and updating our lists. It seemed to work for 2-3 weeks, and now I am back to square one it seems.


Userlevel 5
Badge +18

@HannahfromMDK hey, have you perhaps tried sending text only emails to this audience, and perhaps asking them to reply to your emails?

I understand the emails are bouncing however text-only emails might pull through, especially if they are using non-spammy verbiage and if they are sent as if they were a normal business email.

This might help push through to your Comcast audience and reset the spam filter algorhythm in your favour. 

Further to this, here’s an EXHAUSTIVE list of conditions that need to be met in order to successfully deliver to Comcast:

1. Conform to the standards
All email must comply with all relevant RFCs.

2. Ensure your rDNS is correct
Comcast performs an rDNS check on the IP of the sending server. If the sending server does not have a PTR record and a MX or an A record set up properly, the connections will not be accepted.

3. Stay off DNSBL lists
Comcast consults several DNSBLs including Spamhaus Zen, and ReturnPath. Being listed on any reputable, widely used, DNSBL is likely to cause your email to be blocked not only by Comcast, but other ISPs as well.

4. Keep your distribution lists clean
A large number of undeliverable emails sent to Comcast will result in a sending IP being blocked. All “Not Our Customer” NDNs should be treated as an unsubscribe request.

5. Practice good abuse management
Enforce AUPs and mitigate compromised accounts and machines. Dictionary or Directory Harvest attacks are treated seriously and will quickly lead to the sending IP being blocked.

6. Keep it Static
Dynamic IP space is a common haven for compromised machines used in bot networks. As a result, in keeping with industry best practices, Comcast does not accept mail from dynamic IP space. If email appears to be coming from dynamic IP space then it will not be accepted.

7. Keep your IP reputation clean
Overall IP reputation is key to successfully sending to Comcast. If Comcast, or another reputable organization repeatedly receives spam, spyware or other malicious attacks, phishing efforts or other deceptive emails from a certain IP address, Comcast is more likely to block future messages from that IP address. Ensure the overall sending reputation of the sending mail system is as clean as possible.

8. Pay attention to error messages
Most non-delivery notifications Comcast returns contain links to FAQs which explain why the block occurred and instruct in how to remediate the issue. Take the time to review any errors; it is likely they hold the key to a quick remediation.
Visit our error messages page.

9. Watch the sending limits
Comcast allows 25 simultaneous connections per sending IP address.
Comcast allows 100 recipients per message.
Comcast allows a throttle rate based on your sender reputation and authentication.

Perhaps try to review your “send from” email address inbox to see if there are any error messages from Comcast which you can use to further troubleshoot this, or even get in touch with Xfinity via their Postmaster tool for further help. 

Userlevel 5
Badge +25

Jumping in here as well with some additional context that these kinds of issues are not specific to Klaviyo. Inbox providers are constantly changing their filters and algorithms to determine what is and isn’t spam, and they all have different definitions. Particularly now after the Yahoogle updates, the rules only get tighter and tighter. 


Mass shared domains like Comcast are extra sensitive because it only takes a small fraction of address to mark your domain as spam before they lock you out entirely for everyone, even if other addresses engage regularly. 


Check out this thread with more info on how to repair your reputation with a specific inbox provider:


Userlevel 3
Badge +9

We also saw significant bounces since the Feb.1 changes, that resulted in a steep downward move to our deliverability score for a couple accounts.. Comcast in the US and BigPond in AU were the main domain providers who were blocking emails, resulting in high bounce rates.


We were able to work with Klaviyo to reverse the deliverability issue thankfully. They reached out to these providers for us and got us removed from their blacklists after a few send tests from us to them to show that we are sending legitimate emails.


In addition to removing these domain names from campaigns as others have said above, we also added a filter to all flows “Has not bounced ever over all time”. This has helped our flow bounce rates improve significantly, since Klaviyo’s default is to wait for 7 bounces before they suppress contacts. 


Klaviyo also confirmed that they have seen a significant increase in bot traffic in a lot of accounts since Feb. 1, and recommended we switch to a double opt-in email subscription process.. which we did for 2 accounts that were bad, and this helped tremendously, but also reduced our sign ups and revenue from new sign ups significantly, so once the deliverability scores got better - we have reverted back to single opt-in and have seen our revenue and sign up rates back to normal.