There is something incredibly frustrating about spending time and effort building an email campaign, pressing “send” and watching emails bounce back. Understanding what caused the low delivery can then seem confusing – was this a data issue? A mail block? A permanent or temporary problem? In this blog post, we will aim to give you a few keys so you feel more confident troubleshooting email bounces.
What’s a bounce?
From a technical standpoint, sending an email is a conversation between two servers: the sending server and the receiving server.
When you launch an email campaign, receiving servers return automated responses to confirm whether they’re accepting your message or rejecting it. If the message can’t be delivered, the receiving server issues a delivery failure report – commonly known as a bounce. The full message returned by the receiving server explains the root cause of the failure.
Typically, a raw bounce message includes:
- A status code defined by the RFC 3463 protocol.
- A text description detailing the specific delivery issue.
Sounds simple enough, right? However, as with most concepts in email deliverability, things get more nuanced once you look under the hood. Understanding how bounces work and how to handle them is critical for troubleshooting delivery issues and maintaining a strong sender reputation.
The possible reasons for email bounces
There are countless reasons an email might bounce, and the real challenge stems from a lack of industry standardization: every mailbox provider and recipient network returns its own unique bounce messaging.
For example, if a recipient’s inbox is full, different providers might report:
- Gmail: 452-4.2.2 The recipient’s inbox is out of storage space
- Yahoo: 552 5.2.2 This message could not be delivered because the recipient’s mailbox is full.
- iCloud: 552 5.2.2 …@… user is over quota
Permanent vs. Temporary failures
Delivery failures fall into two primary categories based on their duration:
- Temporary issues: the receiving sender cannot accept your email right now, you might want to retry sending.
- Permanent issues: the receiving server is not accepting your email now, no point retrying.
As a general rule, 4xx status codes signal a temporary issue, indicating that the sending server should retry delivery later. 5xx status codes signal a permanent error, telling the server to stop trying.
However, interpreting these codes isn’t always straightforward. Mailbox providers vary widely in how precise – or vague – their descriptions are. Compare Gmail’s granular responses to the eccentric messages returned by smaller servers:
Gmail (Detailed):
452-4.2.2 The recipient’s inbox is out of storage space.
550-5.2.1 The email account that you tried to reach is inactive.
552-5.2.2 The recipient’s inbox is out of storage space and inactive.
Smaller providers (Cryptic!):
550 Go away.
421 Goodbye
554 5.7.1 …@… Sender address rejected: nope
To add further complexity:
Out-of-band bounces happen when the receiving server may initially accepts a message and rejects it later.
Mailbox providers frequently introduce new codes. For example, when Gmail updated its bulk sender requirements, it introduced specific authentication failure codes, such as:
550-5.7.27 Your email has been blocked because SPF authentication didn’t pass for this message. Gmail requires all bulk email senders to authenticate To set up SPF for your sending domains, visit https://support.google.com/a?p=setup-spf. To learn more about Gmail requirements for bulk senders, visit https://support.google.com/a?p=sender-guidelines.
How ESPs categorize raw bounces
To help senders make sense of raw server responses, Email Service Providers (ESPs) group these messages into actionable categories. Each ESP uses its own proprietary logic to process and categorize raw error strings.
In Iterable, raw bounce messages are processed and assigned distinct categories to streamline deliverability analysis. In the campaign analytics dashboard (under Messaging Insights > Delivery), senders can view high-level categorical breakdowns or download raw bounce event files for deeper investigation.
Every bounce category falls under one of two core umbrella terms:
- Hard Bounces: Iterable classifies an event as a “hard bounce” only when a mailbox provider explicitly rejects an email because the recipient address doesn’t exist. These invalid addresses are automatically unsubscribed from future sends from the associated mail channel.
- Soft Bounces: every other rejection type falls under “soft bounces”. Because these issues may be temporary, soft bounce addresses are not automatically suppressed.
💡 Migration tip: When switching ESPs, always export your historic hard bounce suppression lists and import them into your new sending platform before launching your first campaign.
You can find more information on how Iterable categorizes bounces here and see a list of all the bounce categories available here.
How to handle repeat soft bounces
Suppressing hard bounces is obvious – sending emails to non-existent addresses mimics spammer behavior and damages your sender reputation. Managing soft bounces, however, requires a deliberate strategy.
When an email soft bounces, Iterable attempts redelivery over a defined window. If delivery continues to fail, the message is ultimately marked as bounced. The danger lies in repetitive soft bounces – addresses that fail send after send without resolution.
For example:
- Full inboxes: a temporarily full inbox is normal. However, if an inbox remains over quota for weeks or months, the owner has likely abandoned the account and you should stop sending to them. Mailbox providers sometimes convert abandoned accounts into spam traps to catch unhygienic senders.
- Policy blocks: mailbox providers can temporarily block your emails – for example, if they have concerns over your sending behavior. However, other domains, such as higher education and corporate domains often enforce strict security policies rejecting all external mail. If a recipient domain continuously blocks your messages, sustained attempts could degrade your domain reputation.
- Deprecated infrastructure: if a target mail server remains continuously offline, the domain may be decommissioned.
Ignoring repeated soft bounce signals demonstrates poor list hygiene to receiving servers, which can lead to severe blocklisting, widespread domain blocks, and spam folder placement.
Spamhaus, one of the world’s leading threat intelligence networks, suggests a simple benchmark: “Have you successfully delivered an email to this mailbox in the past six months?”
Customizing Suppression Logic in Iterable
Rather than applying rigid blanket suppression rules across all senders, Iterable gives you the flexibility to build custom suppression rules tailored to your sending cadence and bounce types.
For example, you might want to unsubscribe users that keep returning Mailbox Full errors after 4 weeks.
- If you send daily: set a rule to suppress users who generate 30 “Mailbox Full” bounces within 30 days.
- If you send weekly: set a threshold of 4 “Mailbox Full” bounces within 30 days.
Engagement guardrails: to avoid any false positives, you could require that candidates for suppression have generated zero opens or clicks during the target 30 days window.
You can also implement a quarantine strategy: temporarily remove repetitive soft-bouncers from active mailings for 30 days, re-test delivery once, and permanently unsubscribe them if the failure persists.
Final Thoughts
Every bounce message is direct feedback from a receiving server. High-deliverability senders consistently listen to this feedback and adjust their lists accordingly.
In over a decade of working in email deliverability, I’ve learned that most inbox placement problems stem from data quality issues. If you’re experiencing delivery or reputation dips, start by examining your bounce metrics. Dive into your raw error data, refine your categorization, and implement a robust bounce suppression logic – it is often the single most effective step towards restoring your domain health and improving overall program performance.
Want to learn more about optimizing your bounce suppression rules? Contact your Customer Success Manager or Account Representative to learn how our Deliverability Team can help.
Additional resources
https://support.iterable.com/hc/en-us/articles/208182926-Email-Bounces
https://support.iterable.com/hc/en-us/articles/28534979433236-Email-Bounce-Event-Properties
https://support.iterable.com/hc/en-us/articles/207907633-Unsubscribing-Users-with-Email-Soft-Bounces
