High email bounce rate? Burned domain or bad list
A high email bounce rate means a bad list or a burned domain, and the fixes are opposites. Read the codes, where they cluster and when they start.
Karim MFounder, SendingOS · 5 min read
A high email bounce rate sends most teams straight to one of two fixes: swap in fresh inboxes, or throw out the list. The trouble is that each fix makes the other problem worse. Fresh inboxes sending to a bad list burn too. A clean list sent from a burned domain still lands in spam, and now you've spent the list.
So before you touch anything, work out which one you have. A burned domain and a bad list leave different marks in your bounces, and you can usually tell them apart in a few minutes.
Read the bounce, not the bounce rate
A bounce rate is one number made of very different events. The bounce message itself says which kind you got. It starts with a three-digit reply code, and most servers add an enhanced code after it.
Don't stop at 550. It only means "rejected, permanently", and it shows up on a dead address and a spam block alike. "550 5.1.1" and "550 5.7.1" are opposite problems. The enhanced code and the text after it tell them apart:
5.1.x: the address doesn't exist. 5.1.1 is "bad destination mailbox": the mailbox was never there, or it's been deleted. That's a list problem. The receiving server never got as far as judging you.
5.7.x: the server refused you. 5.7.1 means delivery wasn't authorized, and the text usually says why: a block list, a policy, your reputation. Gmail's 5.7.26 means the message failed authentication (SPF, DKIM or DMARC). These are sender problems.
4.x.x: try again later. A full mailbox (4.2.2) is about the recipient. A deferral for an "unusual rate of unsolicited mail" (Gmail's 4.7.28) is about you, and it's an early warning, not noise.
The 550s that mean spam
Watch these most closely. They're the receiving server telling you it has judged your mail as spam:
Gmail: "550 5.7.1 … this message is likely unsolicited mail", or a rejection that mentions the very low reputation of the sending domain or IP.
Microsoft: "550 5.7.1 … part of their network is on our block list (S3150)", or "Message rejected as spam by Content Filtering".
Any server: "blocked using" followed by a block list's name, such as Spamhaus.
They split two ways. A rejection that names your reputation or a block list is about the domain or the IP: that's a burned domain, or one on its way. A rejection for the content of a message can hit a healthy domain too, so check the copy and the links before you blame the domain. If the same message is rejected as spam from two different domains, it's the message.
Raw bounce emails are tedious to read one at a time, which is why most teams never do. In SendingOS, every bounce is sorted into hard, soft or block, with its reason and SMTP code shown on the conversation in the inbox, so a spam block never hides in the same number as a dead address. To look up any single code, use our bounce code library.
The rough rule: a pile of "550 5.1.1 user unknown" is a list. A pile of 550s that say spam, reputation or block list is a sender.
Where the bounces cluster
The second question is where the bounces live.
A bad list bounces everywhere it goes. If one lead source or one campaign is bad, its bounces show up on every domain and inbox that sends to it. Your other campaigns, sending from the same domains, look normal.
A burned domain bounces everything it sends. The blocks pile up on one domain's inboxes, across every campaign and list that domain touches. Other domains sending the same campaign to the same kind of leads are fine.
So split your bounces two ways: by list or campaign, and by sending domain. Whichever way they line up is your answer.
When the bounces start
Timing tells you a lot too.
A bad list hits hardest on the first step. Invalid addresses bounce the first time you write to them and drop out of the sequence, so the follow-ups bounce less. If step one bounces and step two is clean, look at the list.
A burning domain gets worse over time. Replies thin out first, then deferrals appear, then hard blocks. It hits new leads and follow-ups alike, because the receiving server is judging the sender, not the address. If bounces climb day after day on the same domain while the list hasn't changed, look at the domain.
Check the warmup
One more test: look at the domain's warmup mail. A bad list can't touch warmup, because warmup never goes to your prospects. If the domain's warmup mail has started landing in spam too, the problem travels with the domain.
High email bounce rate: list or domain at a glance
Signal | Bad list | Burned domain |
|---|---|---|
Typical code | 550 5.1.1 user unknown | 550 5.7.1 spam, reputation or block list; 5.7.26 authentication |
Bounces line up by | List or campaign | Sending domain |
Other domains, same list | Bounce too | Fine |
Same domain, other lists | Fine | Bounce too |
Over time | Worst on step one | Worse day by day |
Warmup | Unaffected | Starts landing in spam |
If it's the list
Stop sending to it. Pause the campaign or pull the list. Every step that reaches a dead address makes the domain look careless.
Find out where it came from. Guessed address patterns, old exports and scraped lists are the usual suspects.
Rebuild the addresses instead of guessing them. In SendingOS, the Airscale waterfall tries source after source until it finds a real email for the lead.
Leave the domain alone. It isn't burned. Swapping in new inboxes here only spends them.
If it's the domain
Pause its inboxes now. Every extra send to "see if it's better" makes it worse. In SendingOS, domain health rules do this on their own: when a domain drifts, its inboxes pause before the whole campaign suffers.
Check authentication first. If the blocks say 5.7.26 or mention SPF, DKIM or DMARC, it may not be reputation at all, just a broken DNS record. Fix it and the blocks can stop as fast as they started.
Rest it on warmup only. Keep warmup running, keep campaigns off, and bring the domain back slowly once its warmup mail lands in the inbox again. There's no fixed number of days; let placement tell you.
Fix what burned it before you replace it. If copy, links or volume burned the domain, moving the same campaign to fresh domains burns those too.
A bad list turns into a burned domain
The two problems don't stay separate for long. Mailbox providers read a lot of mail to addresses that don't exist as a sign of a careless sender. Keep sending a bad list and the "user unknown" bounces turn into blocks on the domain. Catch it early and it's a list problem. Catch it late and it's both.
The habit that saves you is boring: read your bounce reasons every day, split by list and by domain. In SendingOS, bounced threads carry a Bounced label in the inbox, so that daily check is one filter, not a hunt.
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Book a callQuestions
What causes a high email bounce rate?
Usually one of two very different things: a bad list, where the addresses don't exist (550 5.1.1), or a burned domain, where servers refuse the sender (550 5.7.1 spam or block-list rejections). Read the codes, then check whether the bounces line up by list or by sending domain.
Does a 550 bounce mean my domain is burned?
Not on its own. 550 only means the message was rejected permanently. 550 5.1.1 is an address that doesn't exist, which points at the list. 550 5.7.1 with spam, reputation or a block list in the text is the server refusing you, which points at the domain.
What's the difference between a hard bounce and a block?
A hard bounce (a 5.1.x code such as 5.1.1) means the address doesn't exist, which points at the list. A block (usually a 5.7.x code) means the receiving server refused the sender because of reputation, a block list or failed authentication, which points at the domain.
Can a bad list burn a domain?
Yes. Mailbox providers read a lot of mail to addresses that don't exist as a sign of a careless sender. Left running, a bad list turns into blocks on the domain that sent it, so stop the list as soon as the user-unknown bounces show up.
How long should a burned domain rest?
There's no fixed number of days. Pause its campaigns, keep warmup running, fix whatever burned it, and bring it back gradually once its warmup mail lands in the inbox again.
