Hey friends,
Ever had a perfectly good subscriber go dark—and wondered if they vanished, ghosted, or got swallowed by Gmail?
Today we’re cracking open one of the most misunderstood parts of email deliverability: bounces.
They’re not just delivery hiccups. They’re signals.
Signals that tell you when to retry, when to suppress, and when you’re about to tank your sender reputation if you don’t act fast.
Let’s decode the bounce.
Hard vs. Soft Bounces
Before you go mass-deleting anyone with a failed delivery, let’s pump the brakes. Not all bounces mean the door’s permanently shut.
Some are just a "try again later."
Knowing the difference is key to keeping your deliverability (and sanity) intact.
❌ Hard Bounce = Permanent Failure
Think of it as a brick wall. This message isn’t getting through—ever.
Why it happens:
Email or domain doesn’t exist
Mailbox was deleted
You’ve been blocked due to a complaint
What to do:
Suppress or delete immediately. Do not retry.
⚠️ Soft Bounce = Temporary Issue
The door’s closed… for now. Try again later.
Why it happens:
Mailbox is full
Server problems
Message is too large
Temporary spam filter caught it
What to do:
Retry 2–3 times over a few days. If it keeps bouncing, consider suppressing.
🧾 Bounce Codes 101
Here’s your cheat sheet for interpreting those mysterious numbers:
Code | Type | Meaning |
|---|---|---|
550 | Hard | Mailbox not found / blocked |
552 | Soft | Mailbox full / over quota |
554 | Hard | Spam-related rejection |
421 | Soft | Temporary system issue |
4xx | Soft | Retryable errors |
5xx | Hard | Permanent failures |
Don’t just skim them—these are the breadcrumbs leading back to your inbox.
👥 What to Do With Bounced Subscribers
Subscriber Type | Action |
|---|---|
Hard bouncers | Suppress right away. No second chances. |
Repeated soft bouncers | Retry 2–3 times, then suppress if it’s not improving. |
Valid users (temp issues) | Handle with bounce-specific logic (see below). |
Now let’s get into the weeds—because Gmail and Yahoo always bring the weird.
📛 Special Case: Yahoo TS04 Responses
Ever seen this?
421 4.7.0 [TS04] Messages temporarily deferred due to user complaints
It’s not a hard bounce—it’s throttling.
Why?
High complaint rates
Sudden spikes in volume
Sender reputation issues
✅ How to handle:
Throttle sends to Yahoo domains (yahoo.com, aol.com, verizon.net)
Send only to recently engaged users
Retry slowly—these are temporary deferrals
Use Yahoo Postmaster Tools to monitor complaints
Segment Yahoo users separately and rebuild trust gradually
🚫 Do not delete these subscribers. They’re still valid. Cutting them is a loss.
📬 Special Case: Gmail 552 “Mailbox Full”
Ever see this one?
552 5.2.2 The email account that you tried to reach is over quota.
It’s a Gmail soft bounce caused by storage overuse—often Drive, Photos, and Gmail combined.
These users are alive. They just need to clean out their inbox.
✅ How to handle:
Retry over 30–45 days
Suppress if still bouncing after 60 days
Check again monthly—they may free up space
If they were high-engagement, give them more time before suppressing
🧠 Pro Tip: Suppress. Don’t delete. They may come back to life.
SMTP Errors and Codes by ISP
Here are links for detailed breakdowns of each bounce reason by the major ISPs.
BEFORE YOU GO
3 Ways I Can Help
🔹 Smart Feed – A steady flow of engaged, high-intent subscribers delivered directly to your list every day.
🔹 Smart Pixel – Turn anonymous website visitors into verified email subscribers—without forms or friction.
🔹 Smart Delivery – Keep your emails out of spam and in the inbox with expert-driven deliverability solutions.
✅ Final Thought
Every bounce tells a story.
Sometimes it’s “this person ghosted you.”
Other times it’s “you’re sending too fast to a sensitive domain.”
Or it’s just “they ran out of Gmail storage because of 6 years of cat photos.”
Either way—don’t ignore it.
Handle bounces with precision and purpose.
Because a clean list and sharp deliverability are what keep your emails in front of real people—not in the spam folder graveyard.
To the inbox and beyond,
Chris Miquel
PS: Most senders obsess over opens and clicks—but ignore the signals that quietly sabotage inboxing. If you haven’t looked at your bounce reports in the last 30 days… do it today. Your sender reputation depends on it.
