What does a hard bounce user unknown [5.0.0 SMTP reply matched bounce-rcpt pattern rule] mean?
Michael Ko
Co-founder & CEO, Suped
Published 21 Jun 2025
Updated 17 Aug 2025
9 min read
Receiving a hard bounce message that states "User Unknown [5.0.0 (SMTP reply matched bounce-rcpt pattern rule)]" can be incredibly frustrating. When you see this, your first thought is likely that the email address is permanently invalid and you need to remove it from your list immediately. This specific error message, often seen with ESPs like Salesforce Pardot, seems clear-cut, implying a permanent failure.
However, appearances can be deceiving in the world of email deliverability. While a `5.0.0` SMTP status code generally signifies a permanent error (a hard bounce), the additional text, especially the parenthetical SMTP reply matched bounce-rcpt pattern rule, suggests that your ESP has categorized a more specific SMTP reply. The real story behind this bounce message can be quite different, potentially indicating a temporary issue rather than a permanent one. Let's delve into what this message truly means and how to handle it effectively to maintain a healthy email list and strong sender reputation.
Decoding the bounce message
The message "Hard bounce - User Unknown [5.0.0 (SMTP reply matched bounce-rcpt pattern rule)]" is often an interpretation provided by your Email Service Provider. It's not the raw, unadulterated SMTP response directly from the recipient's mail server. ESPs translate the actual SMTP codes and diagnostic messages into their own internal categories to simplify bounce reporting for users. This can be helpful, but it can also mask the true underlying reason for the bounce, especially when the classification is inaccurate.
The critical step here is to request the actual SMTP bounce message from your ESP's delivery logs. This is the precise error code and diagnostic text that the recipient's mail server sent back to your ESP's mail transfer agent (MTA). For instance, a common actual SMTP reply from Gmail for what an ESP might classify as a 5.0.0 user unknown bounce, is actually a 452 4.2.2 The email account that you tried to reach is over quota. Note that real Gmail bounces often conclude with -gsmtp, a crucial detail for identification.
Understanding this distinction is vital because a `4.x.x` code (like `4.2.2`) indicates a temporary issue, not a permanent one. An "over quota" message means the recipient's mailbox is full, but they could clear it out and receive emails again in the future. This is fundamentally different from an "unknown user" (a permanent issue), which typically warrants immediate removal from your mailing list.
Hard bounce versus soft bounce: the crucial distinction
The world of email deliverability broadly classifies bounces into two types: hard bounces and soft bounces. Knowing the difference is paramount for maintaining a clean and effective email list. A hard bounce (or permanent bounce) occurs when an email cannot be delivered due to a permanent reason. This typically means the recipient's email address does not exist, the domain name is invalid, or the mail server has permanently blocked your address or domain. For example, a common hard bounce is "550 5.1.1 Recipient address rejected: User unknown". When you encounter such an error, the address should be removed from your list to protect your sender reputation and avoid future delivery issues. Ignoring these can lead to your emails being marked as spam or your domain appearing on a blocklist (or blacklist).
In contrast, a soft bounce (or temporary bounce) indicates a transient delivery failure. The email address is valid, but the message couldn't be delivered for a temporary reason. Common causes include a full mailbox, the recipient's server being temporarily down, or the message size exceeding limits. Soft bounces usually resolve on their own, meaning the recipient might be able to receive emails again after a short period. Unlike hard bounces, these addresses shouldn't be immediately removed from your list, but rather suppressed or retried later.
SMTP codes provide a standardized way to communicate these delivery statuses. Codes starting with 5.x.x typically denote permanent failures (hard bounces), while 4.x.x codes signify temporary issues (soft bounces). The discrepancy in the example, where the ESP reports a 5.0.0 (a generic hard bounce) but the actual SMTP diagnostic is 4.2.2 (mailbox full, a soft bounce), highlights a critical miscategorization that can lead to erroneous list cleaning. You can learn more about general email bounce error codes at Senderscore's troubleshooting guide.
Misleading categorization
When your ESP reports a bounce, they often map the raw SMTP response to their own defined categories. In the case of "Hard bounce - User Unknown [5.0.0 (SMTP reply matched bounce-rcpt pattern rule)]" with an underlying `4.2.2` (mailbox full) diagnostic, the ESP's rule set incorrectly flags a temporary issue as a permanent one.
Why ESPs miscategorize bounces
ESPs streamline complex SMTP responses into simpler categories for user convenience. However, this simplification can sometimes lead to miscategorization, particularly with generic `5.0.0` bounce codes. While a `5.0.0` response often indicates a fatal or permanent issue, it's a broad category. Without the specific diagnostic information (the dsnDiag field in bounce reports), it’s impossible to know the exact cause. If the underlying `dsnDiag` points to a temporary problem, the ESP's categorization as a hard bounce is incorrect.
ESPs apply their own rules, or bounce-rcpt pattern rules, to translate the technical bounce message into a user-friendly classification. Sometimes, these rules are overly aggressive, classifying certain temporary errors as permanent ones. This could be due to outdated rule sets, a desire to err on the side of caution for list hygiene, or a lack of granular detail in their reporting interface. It's why direct access to the raw SMTP logs is always preferred for accurate troubleshooting.
When dealing with a bounce message like this, it's crucial to insist on seeing the full SMTP reply, especially the diagnostic code. This is what the receiving mail server actually communicated. The ESP's high-level status (dsnStatus) is their interpretation, while the diagnostic message (dsnDiag) is the authoritative detail. Understanding common Gmail SMTP errors and codes can help you better interpret these messages.
The misleading message
ESPs often oversimplify complex SMTP responses, presenting a generic hard bounce code (e.g., `5.0.0`) instead of the specific diagnostic.
Lack of transparency means you might not see the crucial details (like `4.2.2 over quota`) that indicate a temporary issue.
Seeking the truth
Always request the full SMTP bounce log from your ESP to get the exact diagnostic code from the receiving mail server.
Prioritize the diagnostic code (`dsnDiag`) over the ESP's generalized status (`dsnStatus`) for accurate bounce classification.
Impact on deliverability and what to do
The primary impact of miscategorized hard bounces is the unnecessary removal of valuable, engaged subscribers from your mailing list. If an email address is marked as a hard bounce when it was merely over quota, you lose the opportunity to reach that recipient once their mailbox is cleared. This directly affects your engagement metrics, potential conversions, and overall email marketing ROI. It can also cause friction within your organization, especially if you're trying to prove a valid email address is indeed deliverable, as discussed in why a valid email hard bounced.
For `4.x.x` (soft) bounces like "mailbox full" or "server unavailable", the best practice is to temporarily suppress the recipient rather than permanently remove them. Many senders implement a retry logic, attempting to resend the email after a certain period (e.g., 24-48 hours). If multiple soft bounces occur over an extended period, then permanent suppression might be warranted. However, a single instance of a mailbox being full doesn't signify a dead email address.
To mitigate these issues, work with your ESP to understand their bounce classification rules and access the raw SMTP replies. If your ESP's system is consistently miscategorizing soft bounces as hard ones, advocate for changes to their system or adapt your internal processes to account for these discrepancies. Implementing robust bounce handling, whether through your ESP or an external customer data platform (CDP), is essential for good email deliverability. For more in-depth solutions to improve your deliverability, consider exploring our guide on boosting email deliverability rates.
SMTP code
Type
Meaning
Recommended action
4.2.2
Soft
Mailbox full (over quota)
Temporarily suppress; retry later.
5.1.1
Hard
User unknown / bad destination mailbox address
Remove from mailing list.
5.0.0
Generic hard
Unknown issue, often requires diagnostic text to clarify
Investigate diagnostic message; if temporary, suppress.
Views from the trenches
Best practices
Always retrieve the raw SMTP bounce message from your ESP's logs, as this is the definitive indicator of the bounce reason.
Distinguish between hard (permanent) and soft (temporary) bounces by understanding the leading digit of the SMTP code (5.x.x for hard, 4.x.x for soft).
For soft bounces like 'mailbox full', temporarily suppress the recipient for a few weeks to allow them to clear their inbox.
Automate the handling of temporary bounces within your Customer Data Platform (CDP) or email system before sending to the ESP.
Regularly monitor your domain reputation and email deliverability metrics using tools like
Common pitfalls
Relying solely on your ESP's summarized bounce categories without checking the underlying raw SMTP response.
Misinterpreting generic 5.x.x codes as always being hard bounces, especially when the diagnostic text indicates a temporary issue.
Immediately removing subscribers from your list due to a soft bounce, leading to lost engagement and potential revenue.
Assuming that all email addresses ending with the Gmail domain are personal accounts, as many businesses also use Gmail for their domains.
Not having a clear process for re-engaging or re-adding temporarily suppressed subscribers after a reasonable hold period.
Expert tips
A true Gmail bounce message will typically end with '-gsmtp', providing a quick way to identify its origin.
The 'dsnDiag' field in bounce reports provides the actual diagnostic message from the receiving MTA, which is more reliable than 'dsnStatus'.
Gmail accounts share storage across all
logo.url="google.com"
Google services, so a 'mailbox full' can be due to excessive files in Drive or Photos, not necessarily email volume.
Expert view
Expert from Email Geeks says that the displayed bounce message is likely from your ESP, not the actual one from
2023-10-20 - Email Geeks
Expert view
Expert from Email Geeks says that you must obtain the exact SMTP response from your ESP's MTA logs to understand the true bounce reason.
2023-10-20 - Email Geeks
Taking control of your bounce data
The key takeaway when you encounter a "Hard bounce - User Unknown [5.0.0 SMTP reply matched bounce-rcpt pattern rule]" message is to look beyond the surface. This is not always a definitive hard bounce. Your ESP's categorization may simplify the data, but it can also obscure the true nature of the delivery failure. Always prioritize obtaining and understanding the raw SMTP response and its diagnostic details.
By correctly identifying soft bounces, even those mislabeled as hard, you can prevent prematurely removing valuable subscribers. This attention to detail not only improves your list hygiene but also safeguards your sender reputation and maximizes the reach of your email campaigns. For continued learning on managing common bounce errors, review what causes 550 5.1.1 user unknown bounces.
Taking control of your bounce data allows for more nuanced and effective list management, ensuring that your messages reach their intended recipients whenever possible. It's a fundamental aspect of robust email deliverability.
Salesforce Pardot, seems clear-cut, implying a permanent failure.
Gmail for what an ESP might classify as a 5.0.0 user unknown bounce, is actually a 452 4.2.2 The email account that you tried to reach is over quota. Note that real Gmail bounces often conclude with -gsmtp, a crucial detail for identification.
