Do soft bounces affect my sender reputation immediately?

While soft bounces are temporary, persistent soft bounces, especially those categorized as "mailbox full" or reputation-based blocks, can negatively affect your sender reputation. ISPs monitor these rates, and high or consistent soft bounces can signal poor list hygiene or sending practices, leading to reduced deliverability and even email blocklisting .

What is an acceptable soft bounce rate?

An acceptable soft bounce rate typically falls below 2-3%. If your rate is consistently higher than this, it's a sign that you should investigate your email list quality and sending practices. Some ESPs might have specific thresholds at which they take action , like suppressing addresses.

How long should I try to send to a soft bouncing email address?

You should closely monitor addresses that repeatedly soft bounce, especially those receiving "mailbox full" errors. After a few attempts (e.g., 2-3), it's generally best to suppress or remove these addresses from your active mailing list. Continuing to send to them can signal poor list hygiene to ISPs and experts .

Are all types of soft bounces equally harmful?

Different types of soft bounces have varying impacts. A temporary server issue or greylisting typically has minimal impact. However, a "block bounce" (where your email is temporarily rejected due to perceived reputation issues) indicates a more serious problem with your sender score. Similarly, persistent "mailbox full" bounces for the same recipient suggest an inactive account and can also be damaging.

Do soft bounces affect email deliverability and sender reputation? - Sender reputation - Email deliverability - Knowledge base

The question of whether soft bounces impact email deliverability and sender reputation is a frequent topic of debate. Many email marketers and deliverability professionals have encountered conflicting information, leading to confusion about how to best manage these temporary delivery failures. It's a nuanced issue, and the impact can vary significantly depending on several factors.

Unlike hard bounces, which indicate a permanent delivery failure (like a non-existent email address), soft bounces are typically temporary. They signal issues such as a full mailbox, a recipient's server being temporarily unavailable, or the message size exceeding limits. While often resolvable, their persistent occurrence can, over time, send negative signals to Internet Service Providers (ISPs).

Understanding soft bounces and their nuances

Understanding what constitutes a soft bounce is the first step. These aren't always straightforward SMTP error codes, and their meaning can sometimes be ambiguous. A 4xx response usually points to a temporary problem, indicating that the mail server will try to deliver the message again later. This is often the case for a full mailbox or server overload.

The challenge arises when a mailbox full bounce is actually a sign that the email address is effectively abandoned. While technically temporary, if a mailbox remains full for an extended period, it behaves much like a permanent failure. I've observed that some ISPs, like

Gmail, might even use a 5xx code for certain mailbox full scenarios, indicating a permanent issue after initial temporary attempts.

Another type of soft bounce that requires closer attention is a block bounce. These occur when the recipient's server temporarily blocks your email, often due to perceived suspicious activity or reputation issues. While still a soft bounce, this type directly indicates that your sender reputation is facing scrutiny.

It's crucial to differentiate between these types because a temporary server issue is far less concerning than an actual block. Your Email Service Provider (ESP) typically handles retry logic for most soft bounces, but understanding the underlying reason for the bounce is key to effective deliverability management.

Varieties of soft bounces

Mailbox full: The recipient's inbox has reached its storage limit.
Server unavailable: The recipient's email server is temporarily down or offline.
Message too large: Your email exceeds the size limit set by the recipient's server.
Reputation-based block: The recipient's server is temporarily rejecting emails due to perceived low sender reputation.
Greylisting: A temporary rejection mechanism to deter spam.

The link to sender reputation and deliverability

While an occasional soft bounce due to a server hiccup is unlikely to harm your sender reputation significantly, a consistently high volume of soft bounces can certainly raise red flags with ISPs. They indicate that your email list might not be as healthy as it should be, or that your sending practices are causing issues.

ISPs (Internet Service Providers) like

Google and

Yahoo monitor your bounce rates as a key indicator of your sending quality. A high soft bounce rate, especially if it includes a significant number of block bounces, can suggest that you're sending to unengaged or problematic addresses. This can lead to a degraded sender reputation, increasing the likelihood of your emails landing in spam folders or being blocked entirely. As Mailmonitor notes, both types of bounces can harm reputation, with hard bounces being more damaging, but persistent soft bounces are also concerning.

Think of it this way: if a store consistently tries to deliver packages to addresses where the recipient isn't home, or the path is temporarily blocked, it eventually reflects poorly on the store's reliability. Similarly, a high rate of undelivered emails, even if temporary, tells ISPs that your sending practices might be inefficient or target poor-quality lists. This can lead to your IP address or domain being added to a blocklist (or blacklist), further impacting your deliverability.

Temporary soft bounce

Nature: Resolvable issues like a recipient's inbox being full or their mail server being temporarily down. Often self-correcting after a few retries.

Impact on reputation: Minimal immediate impact. ISPs might factor in persistent high rates, but isolated incidents are generally not alarming.

Action required: Monitor trends. If persistent for a specific address, consider removing it from your active list.

Persistent soft bounce

Nature: Repeated soft bounces to the same address, often indicating a stagnant or abandoned mailbox, or consistent temporary blocks from an ISP.

Impact on reputation: Significant. High rates signal poor list hygiene or issues with your sending practices, potentially leading to lower

deliverability, as Klaviyo notes.

Action required: Aggressively remove these addresses. They can hurt your sender score.

When soft bounces become a problem

While soft bounces are generally less severe than hard bounces, their cumulative effect can be detrimental. Many ESPs have a tolerance threshold for repeated soft bounces to the same address. Once this threshold is met, the address is typically suppressed, effectively converting a persistent soft bounce into something treated like a hard bounce.

This threshold varies between providers. Some might allow a few soft bounces, while others might suppress after a single mailbox full bounce, recognizing that such addresses are unlikely to become active again quickly. It's important to understand your ESP's specific policy regarding over-quota mailboxes and soft bounces.

A general rule of thumb I've observed is that if your soft bounce rate for any given campaign is consistently above 2%, it's a strong indicator of an underlying issue that needs addressing. While not as immediately damaging as a high hard bounce rate (which can trigger account suspensions at 3-5%), a persistent soft bounce rate suggests a degrading list or content issues that could lead to deliverability problems.

Bounce type	SMTP code category	Typical cause	Impact on deliverability	Action
Mailbox Full	4xx or 5xx	Recipient's inbox at capacity. Can be temporary or indicate abandonment.	Low to medium. Repeated bounces to the same address can hurt.	Remove after a few tries (e.g., 2-3). Review your list hygiene.
Server Unavailable	4xx	Recipient's server offline or overloaded. Resolves on its own.	Very low, unless it becomes a consistent, widespread issue.	ESP will retry. Monitor overall soft bounce rate trends.
Message Too Large	4xx or 5xx	Email size exceeds recipient server's limits.	Low. Indicates content or sending practice issue, not necessarily recipient quality.	Reduce email size, optimize content. Consider suppressing if persistent for specific addresses.
Reputation Block	4xx	Recipient server temporarily blocking due to perceived spam or low sender reputation.	Medium to high. Directly reflects poor sender health.	Investigate content, sending volume, list quality, and authentication.

Strategies for managing soft bounces

Effective management of soft bounces is crucial for maintaining a strong sender reputation and ensuring high email deliverability. The goal isn't just to reduce the bounce rate, but to understand its underlying causes and address them proactively.

A key strategy is to periodically clean your email list and identify patterns in soft bounces. For instance, if you see a high number of mailbox full bounces, it could signal that parts of your list are inactive or stale. Regularly removing unengaged subscribers and maintaining strong list hygiene is paramount to minimizing bounces.

Best practices for managing bounces

Implement double opt-in: Confirms subscriber intent, leading to higher quality email lists.
Regular list cleaning: Remove inactive subscribers and addresses that consistently soft bounce.
Monitor bounce categories: Pay attention to the types of soft bounces to identify root causes, like server issues or reputation blocks.
Segment inactive users: Try re-engagement campaigns for those showing signs of inactivity, but be ready to remove them.
Check email content: Ensure your messages are not too large or triggering spam filters, which can lead to soft blocks.

Views from the trenches

Best practices

Actively monitor all bounce types, not just hard bounces. Look for trends in soft bounce categories.

Segment your audience and send to engaged subscribers more frequently to maintain list hygiene.

Use email validation services before sending to new lists to minimize initial bounce rates.

Implement a consistent process for removing addresses that repeatedly soft bounce after a few attempts.

Common pitfalls

Ignoring soft bounces because they're 'temporary' can lead to hidden reputation damage over time.

Failing to understand the specific type of soft bounce, which limits your ability to address the root cause.

Relying solely on your ESP's bounce handling, without proactively cleaning your list.

Continuing to send to addresses that consistently return 'mailbox full' errors; they're often abandoned.

Expert tips

If soft bounces include block bounces, it's a strong indicator of underlying sender reputation problems.

Not all soft bounces are equally harmful. A full mailbox might not be as bad as a reputation-based block.

ISPs like Gmail count everything towards storage quotas, so 'mailbox full' can mean more than just email.

Some 'mailbox full' addresses might eventually become active again, but the risk of keeping them is often too high.

Expert view

Expert from Email Geeks says that the impact of soft bounces heavily depends on the ISP and the specific type of soft bounce, and that ESPs often don't categorize them well enough.

2023-06-23 - Email Geeks

Expert view

Expert from Email Geeks says that if soft bounce rates are unusually high, perhaps above 2%, this is likely a negative sign for most senders, suggesting other metrics might offer a clearer picture of reputation.

2023-06-23 - Email Geeks

The bottom line on soft bounces

While soft bounces are not as immediately catastrophic as hard bounces, they are far from harmless. A low volume of temporary soft bounces is normal and generally handled by ESPs, but persistent or high rates of soft bounces, particularly those indicating reputation-based blocks or long-abandoned mailboxes, absolutely impact your email deliverability and sender reputation over time.

Proactive list hygiene, understanding the nuances of different soft bounce types, and diligently monitoring your bounce rates are essential. By doing so, you can ensure your emails consistently reach the inbox and maintain a strong, positive sender identity.