What causes delivery errors when no hard bounces are seen in the ESP?
Michael Ko
Co-founder & CEO, Suped
Published 10 Jun 2025
Updated 17 Aug 2025
7 min read
It can be perplexing when you see a spike in delivery errors reported by your mail system, yet your Email Service Provider (ESP) shows no corresponding hard bounces. This scenario is more common than you might think and points to underlying issues that don't always trigger the definitive permanent failure of a hard bounce.
Hard bounces signify an email address is permanently unreachable, such as an invalid or non-existent address. ESPs typically remove these from your lists immediately. However, many other types of delivery issues are temporary or fall into categories that don't register as a hard bounce within your ESP's reporting, even if the email never actually reaches the inbox.
Understanding these nuances is crucial for accurate email deliverability analysis and maintaining a healthy sending reputation. The lack of a hard bounce doesn't necessarily mean successful delivery, but rather that the problem isn't a permanent recipient-side issue.
The nature of temporary delivery failures
Soft bounces and deferrals are the most frequent culprits when delivery errors occur without hard bounces. A soft bounce indicates a temporary delivery failure. This could be due to a recipient's mailbox being full, the receiving server experiencing temporary issues, or the email message size exceeding limits. For more on this, Postmark has a guide on email bounces.
ESPs are designed to retry sending emails that result in soft bounces. The number of retries and the duration (often 24 to 72 hours) vary by provider. If, after exhausting all retries, the email still cannot be delivered, the ESP might eventually classify it as a soft bounce and stop further attempts. However, if the issue is a deferral (a specific type of soft bounce), your ESP might report it simply as a delivery error rather than a bounce, especially if the message is still in the retry queue.
These deferrals often appear in your ESP's logs with codes like 4xx, indicating a temporary issue. The email hasn't been permanently rejected, so it doesn't meet the criteria for a hard bounce, but it also hasn't been successfully delivered. This can lead to a discrepancy where your ESP shows delivery attempts or generic errors, but not an outright bounce.
Bounce Type
Description
ESP Reporting
Hard bounce
Permanent delivery failure, e.g., invalid email address or domain.
Reported as a hard bounce, email usually suppressed immediately.
Soft bounce
Temporary delivery failure, e.g., mailbox full, server unavailable.
Often retried by ESP, may be reported as a soft bounce after retries fail.
Deferral
Temporary rejection, server busy or sender reputation low.
May show as a generic delivery error or still in queue. Not necessarily a 'bounce' per ESP reporting.
Silent drop/block
ISP accepts the mail but filters it to spam or discards it without notification.
Not reported as a bounce; appears as 'delivered' by ESP.
Hidden blocks and filtering
Even if your ESP isn't reporting hard bounces, your emails could be encountering blocks or filtering that prevent them from reaching the inbox. Internet Service Providers (ISPs) and mailbox providers (like Gmail or Outlook) employ sophisticated filtering systems that might silently drop messages or divert them to spam folders without generating a bounce notification. This is often the case when your sending IP or domain is placed on a blocklist (or blacklist).
Greylisting is another common mechanism where an ISP temporarily rejects an email from an unrecognized sender and requests the sending server to retry. If your ESP isn't configured to retry appropriately, or if the receiving server's greylisting policy is aggressive, the email might eventually time out and fail without a hard bounce. Similarly, hitting spam traps can lead to silent drops and reputation damage that manifests as delivery errors.
Content-based filtering can also cause emails to be delivered to the spam folder or silently discarded. This happens when the email's content (links, images, keywords) triggers spam filters, even if the address is valid. While this isn't a bounce, it's a delivery failure from an inbox perspective. If you suspect this, you might also be seeing emails not appearing in the inbox despite successful ESP delivery.
Identifying hidden blocks
Check your sending IP and domain against major blocklists (blacklists). Even if your ESP doesn't report it as a hard bounce, a blocklist entry can significantly impact delivery, causing silent failures. Use a blocklist checker regularly.
Monitor your sender reputation using tools like Google Postmaster Tools. A drop in reputation can lead to increased deferrals and spam folder placement without clear bounce notifications.
Technical and infrastructure issues
Beyond soft bounces and blocklisting, technical issues can also cause delivery errors that don't manifest as hard bounces. For example, problems with a recipient's mail server, such as being offline, misconfigured, or simply overloaded, can lead to temporary rejections. These are often categorized as deferrals by your ESP but might not convert to a hard bounce if the recipient server eventually recovers or your ESP's retry window expires without a definitive permanent error code. This is a common reason some delivery failures are not marked as bounces.
DNS issues, particularly with the Mail Exchanger (MX) records of the recipient domain, can also cause delivery failures. If an MX record is improperly configured or unreachable, the sending server won't know where to send the email. This might lead to deferrals that eventually fail without a hard bounce, especially if your ESP isn't equipped to interpret these specific DNS-related failures as permanent hard bounces.
Furthermore, misconfigured or missing email authentication records like SPF, DKIM, and DMARC can lead to emails being rejected or quarantined by receiving servers. While a DMARC policy set to 'reject' (p=reject) might result in a bounce, a policy set to 'quarantine' (p=quarantine) or missing/improper records might cause the email to be sent to spam or discarded without a bounce notification. This means it's essential to monitor your DMARC reports for these issues.
Subscriber list hygiene and engagement
Good list practices
Regular cleaning: Remove inactive or unengaged subscribers periodically to maintain list quality.
Permission-based lists: Ensure all subscribers have explicitly opted in to receive your emails.
Engagement segmentation: Target engaged segments to improve overall deliverability and sender reputation.
Bad list practices
Ignoring soft bounces: Failing to address temporary issues can lead to persistent delivery problems.
Purchased lists: Sending to unverified addresses increases the risk of hitting spam traps and blocks.
Infrequent sending: Long gaps can lead to increased inactive addresses and more soft bounces.
The quality and engagement of your recipient list play a significant role in deliverability, even when hard bounces aren't being reported. Sending emails to inactive or unengaged subscribers can negatively impact your sender reputation. ISPs monitor engagement signals (opens, clicks, unsubscribes, spam complaints) and will gradually route emails to the spam folder or silently drop them for domains with poor engagement. This results in emails going to spam instead of bouncing.
Furthermore, a sudden spike in email volume or sending to a cold, unverified list can trigger ISP throttling or temporary blocks (deferrals) that are not reported as hard bounces. This is often the case during IP warming or migrations if not handled carefully.
Ultimately, the cleanliness of your list directly influences your sender reputation. A high percentage of low-engagement recipients can lead to ISPs viewing your mail as less desirable, resulting in more emails not being received or routed to spam, without your ESP ever logging a hard bounce.
Views from the trenches
Best practices
Actively monitor ESP logs for deferral reasons, not just hard bounces, to catch temporary issues.
Segment your audience based on engagement and send more frequently to highly engaged users.
Implement a consistent list cleaning process to remove inactive subscribers who may cause silent filtering.
Regularly check your domain and IP reputation using postmaster tools and blocklist checkers.
Ensure proper email authentication (SPF, DKIM, DMARC) is set up and monitored for alignment issues.
Common pitfalls
Assuming 'delivered' status from ESP means emails reached the inbox and were seen by recipients.
Ignoring recurring soft bounces, as they can eventually lead to reputation damage and silent filtering.
Sending large volumes to unengaged segments, which triggers ISP throttling and spam placement.
Not reacting to 4xx SMTP errors, which indicate temporary blocks that can become permanent failures.
Failing to review DMARC aggregate reports for authentication failures that lead to silent rejections.
Expert tips
Utilize seed testing services to check inbox placement across various mailbox providers.
Pay close attention to SMTP response codes beyond just hard bounces, especially 421 deferrals.
If migrating ESPs, adhere strictly to the IP warming schedule to avoid sudden deferrals.
A sudden spike in 'Mailbox full' deferrals, especially for major ISPs, often signals underlying reputation or list issues.
Sometimes, what appears as a delivery error is simply the email sitting in the queue during the retry period.
Expert view
Expert from Email Geeks says often, these delivery errors are actually deferrals, where the email is temporarily held but not immediately rejected.
2024-10-15 - Email Geeks
Marketer view
Marketer from Email Geeks says the email might still be sitting in the queue and could eventually be delivered, depending on the ESP's retry policies.
2024-10-15 - Email Geeks
Beyond the hard bounce: Actionable insights
In conclusion, delivery errors without hard bounces are typically a strong indicator of soft bounces, deferrals, or silent rejections caused by evolving ISP filtering, sender reputation issues, or technical misconfigurations. It's crucial to look beyond basic hard bounce reports and delve deeper into your ESP's detailed logs and SMTP response codes.
Gmail or 
Outlook) employ sophisticated filtering systems that might silently drop messages or divert them to spam folders without generating a bounce notification. This is often the case when your sending IP or domain is placed on a blocklist (or blacklist).
