Hard Bounce vs Soft Bounce — Reading the Bounce Log

What this is for#

Some of the emails you send come back. The receiving mail server rejects them and sends a little notice — a bounce — explaining why. AcelleMail collects those notices in a Bounce log on every campaign report.

This article shows you how to read that log, tells the difference between a hard bounce (permanent — the address is dead) and a soft bounce (temporary — try again later), and explains exactly what AcelleMail does on its own when each one comes back.

The two categories of bounce#

Every bounce carries a status code from the receiving server. The first digit is all you need to tell the two categories apart:

Hard bounce — codes that start with 5 (5.x.x) A permanent failure. The address doesn't exist, the domain doesn't exist, or the message was rejected outright. Sending again won't help. AcelleMail blacklists the address (more on that below) so you stop wasting sends on it.

Soft bounce — codes that start with 4 (4.x.x) A temporary failure. The mailbox is full, the server is busy, or you've been greylisted. The receiving server usually accepts the message on a later attempt. AcelleMail records the soft bounce but takes no action on the address — it leaves the retrying to the sending server, which is where retries belong.

That single rule — first digit 5 = permanent, first digit 4 = temporary — is how AcelleMail itself classifies every bounce it processes. If a bounce notice has no readable status code, AcelleMail files it as unknown.

Open the bounce log#

From a campaign report, open the Sending logs menu in the report nav and choose Bounce log.

At the top you'll see a one-line reminder: "Emails that bounced back — hard or soft bounces from recipient servers." Each row has four columns:

• Recipient — the subscriber that bounced. Click the email to jump straight to that subscriber's detail page.
• Bounce type — a small badge reading hard, soft, or unknown (the value AcelleMail classified the bounce as). All three use the same badge style — the text is what matters, not the colour.
• Reason — the receiving server's response. When the bounce notice carries a clean status line, you'll see the code right up front (e.g. 550 5.1.1 The email account that you tried to reach does not exist). Click the text to read the full message if it's truncated.
• Date — how long ago the bounce was recorded.

There's a search box above the table — type a recipient address to filter the log down. Most bounces explain themselves: the reason text tells you what happened in plain language right after the code.

Common DSN codes and what they mean#

The codes in the Reason column are DSN (Delivery Status Notification) codes. Here are the ones you'll see most often.

Hard bounces (5.x.x — permanent)#

Soft bounces (4.x.x — temporary)#

A code starting with 5 that mentions authentication or spam filtering (the 5.7.x family) is the one to slow down on. Those bounces are very often about your setup — broken DKIM, an SPF mismatch, or a damaged sending reputation — not a bad recipient address. If you see 5.7.x across many recipients at once, that's a deliverability problem to fix, not a list to clean.

Reading real rows#

Look at the rows in the screenshot above and you can read each one like a sentence — the type badge tells you permanent vs temporary, the reason tells you the specific cause:

• moorevanessa@navarro.com — hard, 550 5.1.1 The email account that you tried to reach does not exist. A dead mailbox. AcelleMail has already blacklisted it.
• clayton61@molina.info — soft, 552 5.2.2 The email account that you tried to reach is over quota. The inbox is full right now. AcelleMail recorded it and left it alone; the receiving server can deliver on a retry.
• conleyricky@day.com — hard, 550 5.7.1 Message rejected as spam by Content Filtering. This one's worth investigating on your side before assuming the address is bad.
• mindy49@hodges.com — soft, 451 4.7.0 Recipient server is rate-limiting your IP. Retry recommended. The receiver is deferring you on purpose — normal, no action needed.
• damon34@taylor.com — hard, 550 5.1.2 The domain has no DNS / no MX records. A dead domain — likely a typo when the contact signed up.
• dustin40@reeves.biz — hard, 550 5.7.7 Message failed DMARC alignment for the From: domain. An authentication problem on your side, not a bad address.

The pattern is always the same: the badge gives you the category, the reason gives you the cause. Read both.

What AcelleMail does automatically#

There is no "bounced" status in AcelleMail and no built-in soft-bounce retry queue. A hard bounce flips the subscriber to Blacklisted — that's the status you'll see on the subscriber and in your blacklisted count. Soft bounces are recorded for your visibility and nothing more.

Why blacklist instead of just unsubscribe? A hard bounce means the address is dead globally, not just unhappy with one list. Blacklisting suppresses it across all your lists at once, which is what protects your sending reputation.

Common issues and what to do#

When an address was blacklisted by mistake#

Because a hard bounce blacklists globally, the way to undo it is to remove the address from your Blacklist, not to "un-bounce" it:

1. Open Blacklist from the sidebar (under the Sending section).
2. Find the address and remove it.
3. Only do this when you have a genuine reason to believe the mailbox is live again (the person re-confirmed, or replied asking to keep getting your emails).

Don't bulk-remove blacklisted addresses without that evidence. Re-sending to addresses that already bounced hard is exactly the behavior that damages your reputation with receiving servers — the damage compounds with each campaign.

DSN codes, decoded#

If you want to read codes you don't recognize, every DSN code has the same three-part shape:

X . Y . Z
│   │   │
│   │   └─ Detail — the specific status (e.g. 1 = mailbox doesn't exist)
│   └───── Subject — the status class (e.g. 7 = security/policy)
└───────── Class — 5 = permanent, 4 = temporary, 2 = success

The Subject digit (the middle one) groups the cause:

So 5.1.1 reads as permanent · addressing · bad mailbox — a dead address. 4.7.0 reads as temporary · security · policy — typically greylisting or rate-limiting. The code tells you the category; the reason text next to it tells you the specifics. Read both.

What to do after#

• If hard bounces are high on a list, run Email list hygiene and verify the list before your next campaign.
• If you're seeing 5.7.x or authentication-flavored bounces, work through the delivery diagnostic checklist.
• Want the full code-by-code reference? See Decoding bounce messages.

Related articles#

• Decoding SMTP error codes and bounce messages
• Email delivery issues — diagnostic checklist
• Sender reputation monitoring
• Email list hygiene — clean your list for better deliverability
• Feedback loops (FBL) setup