What causes soft bounces for a single contact and how should they be handled?
Matthew Whittaker
Co-founder & CTO, Suped
Published 11 Jun 2025
Updated 18 Aug 2025
7 min read
Email soft bounces are a common occurrence for anyone engaged in email marketing or transactional sending. Unlike hard bounces, which indicate a permanent delivery failure, a soft bounce signals a temporary issue. This means the email address is valid, but the message couldn't be delivered at that specific moment. While often temporary, understanding why a single contact might repeatedly soft bounce is crucial for maintaining good sender reputation and deliverability.
When you see soft bounces, it typically means the recipient's mail server temporarily rejected the message. This can happen for various reasons, from a full inbox to server downtime. Identifying the specific cause is the first step toward effective troubleshooting, especially when it affects an individual contact inconsistently, as highlighted by discussions among email professionals.
The key distinction between a soft bounce and a hard bounce is that soft bounces imply a chance for future delivery. However, if a contact frequently soft bounces, it suggests an underlying problem that needs to be addressed to prevent negative impacts on your overall email deliverability.
Common causes of soft bounces
A soft bounce can occur for a variety of reasons, even for a single recipient. Understanding these causes is the first step to mitigating them. One common reason is that the recipient's mailbox is full, meaning they have exceeded their storage limit and cannot receive new messages. This is often a temporary situation that resolves itself once the recipient clears some space. Another frequent cause is that the recipient's server is temporarily unavailable or offline for maintenance. This is outside your control and usually resolves once the server comes back online.
Message size limits are also a common culprit. If your email, including attachments, exceeds the recipient server's allowable size, it will trigger a soft bounce. This can be particularly frustrating when only certain emails to a contact bounce while others go through, suggesting that some of your messages are simply too large. Similarly, certain content within your email might trigger filters on the recipient's server, leading to a temporary block, as some mail systems are configured to loop blocked messages back through their own system.
Rate limiting by the receiving mail server can also cause a soft bounce. This happens when the server detects a high volume of emails from your IP address or domain within a short period and temporarily throttles incoming mail. This is a protective measure to prevent spam and can explain why only some of your emails to a specific contact are bounced, especially if you're sending multiple emails over a short duration.
Understanding SMTP response codes
When an email soft bounces, you usually receive a bounce message with an SMTP status code. These codes provide valuable clues about why the delivery failed. For example, a 4XX code indicates a temporary failure, such as 450 (mailbox unavailable) or 452 (mailbox full). It's essential to consult these codes to understand the specific reason for the bounce. For a deeper dive into common bounce reasons, you can read this guide on email bounces.
Understanding bounce messages
Diagnosing a soft bounce starts with examining the bounce-back message you receive. These messages, often containing SMTP codes, are your primary source of information. A bounce code in the 400 series indicates a temporary issue. For example, 421 might mean the service is temporarily unavailable, while 450 usually means the mailbox is temporarily full or unreachable. Pay close attention to the accompanying text, which often provides more specific context from the recipient's mail server.
It can be confusing when only some emails to a contact bounce back with an error, while others go through. This inconsistency strongly suggests a temporary condition rather than a permanent problem with the email address itself. Factors like network congestion, recipient server load, or dynamic content filtering can cause intermittent delivery issues. If a Microsoft Outlook or Office 365 system is configured to forward emails, and the downstream system has content filters, some messages might get caught in a loop and bounce, while others pass through.
For specific services like Gmail, you might encounter specific soft bounce errors like 4003 or 4004. These often relate to temporary rate limits or content issues and require specific strategies for resolution. Observing patterns over time can help confirm whether the issue is truly transient or if it's pointing to a more systemic problem with that particular recipient's setup or your sending practices relative to that recipient.
Example SMTP bounce codestext
550 5.1.1 Recipient address rejected: User unknown
450 4.2.2 Mailbox full
421 4.7.1 Service unavailable
Strategies for handling single contact soft bounces
When a single contact experiences persistent soft bounces, it's essential to handle them strategically to protect your sender reputation. Most email service providers (ESPs) automatically retry sending soft-bounced emails for a certain period, usually 24 to 72 hours. If the issue resolves, the email will eventually be delivered. However, if the soft bounces persist over multiple sends and for a prolonged duration, it's time to consider a more proactive approach.
Continuing to send to a contact who consistently soft bounces can negatively impact your sender reputation. It signals to receiving mail servers that you're not managing your list effectively, which could lead to your emails being routed to spam folders or even your domain being added to a blacklist (or blocklist). For single contacts experiencing persistent issues, it’s advisable to temporarily suppress them from your sending list. This doesn't mean deleting them, but rather pausing sends to allow time for the recipient's issue to resolve, or for you to investigate further.
The recommended approach for handling soft bounces often involves a threshold. After a certain number of consecutive soft bounces, you should consider the contact as inactive or problematic and suppress them. While there isn't a universally agreed-upon number, a common practice is to suppress after 3-5 consecutive soft bounces. Some platforms, like Mailgun, also provide guidance on this. You can always try to re-engage with them through other channels if available.
Temporary soft bounces
Definition: Indicates a temporary issue preventing email delivery, often resolving within hours or days.
Causes: Full mailbox, server downtime, message too large.
Handling: ESPs typically retry delivery automatically for a set period.
Persistent soft bounces
Definition: Occurs when a contact repeatedly soft bounces over multiple sends and an extended period.
Causes: Misconfigured server, consistent rate limiting, content filtering, or an unmonitored mailbox.
Effectively managing soft bounces for individual contacts is a critical component of maintaining a healthy email program and protecting your sender reputation. While soft bounces are temporary by nature, their persistence can signal deeper issues that warrant attention. Ignoring them can lead to your emails being marked as spam or your domain ending up on a blocklist, even for a valid contact.
The key is to combine automated retry mechanisms with a thoughtful suppression strategy. Continually monitoring your bounce reports and understanding the specific SMTP codes will empower you to make informed decisions. By proactively managing these temporary failures, you can ensure your messages reach their intended recipients more reliably and maintain a strong sender reputation over time.
Views from the trenches
Best practices
Monitor your bounce reports diligently and categorize soft bounces based on the SMTP error codes provided.
Implement an automated retry system through your ESP that attempts delivery for a reasonable period, typically 24-72 hours.
Establish a clear suppression policy for individual contacts who consistently soft bounce over multiple campaigns.
Common pitfalls
Ignoring persistent soft bounces, which can degrade your sender reputation over time and affect overall deliverability.
Failing to analyze SMTP bounce codes, leading to a missed opportunity to understand and address underlying issues.
Continuing to send to contacts who consistently soft bounce without a suppression strategy, potentially leading to blocklisting.
Expert tips
Look for patterns: if only certain types of emails or emails sent at specific times soft bounce, investigate content or timing.
Consider engaging problematic contacts through other channels if email is consistently failing.
Review recipient domain's MX records and any custom configurations if an entire domain shows unusual soft bounce behavior.
Expert view
Expert from Email Geeks says that if a soft bounce error indicates a configuration issue, it might be dependent on a specific receiving MX record or a customer-related condition. A common cause for such an error could be a misconfigured on-premise Exchange server.
2022-04-20 - Email Geeks
Marketer view
Marketer from Email Geeks says that they once investigated mail loop replies from Office 365 hosted servers and concluded the underlying issue was often that the Office 365 system was configured to forward to another system, which then used a filter that blocked the message, creating a loop.
Microsoft Outlook or Office 365 system is configured to forward emails, and the downstream system has content filters, some messages might get caught in a loop and bounce, while others pass through.
