Hotmail/Live email bouncebacks causing errors and failures on LISTSERV

Please note that our forum is a public platform, and we will modify your question to hide your personal information in the description. Kindly ensure that you hide any personal or organizational information the next time you post an error or other details to protect personal data. 

Hi @Jacob Pelley

Thank you for posting your question in the Microsoft Q&A forum.  

First, I’d like to clarify that this is a user‑to‑user support forum. Moderators, contributors, and external Microsoft employees participating here do not have access to backend systems, nor can we directly intervene in Microsoft product features. Our role is limited to providing technical guidance and sharing best practices based on reported issues, requests, or ideas. 

Based on my research, this might relate to Microsoft has been tightening bulk-sender policies over the last couple of years (more aggressive spam filtering, stricter reputation scoring, lower tolerance for lists that look like bulk mail, etc.). Consumer mailboxes (@hotmail.com, @live.com, @outlook.com) are especially strict because they’re free accounts with high abuse potential. 

You can refer via: Strengthening Email Ecosystem: Outlook’s New Requirements for High‐Volume Senders | Microsoft Commu… 

Microsoft uses a two‑stage model to manage performance and spam control for high‑volume or bulk email: 

During the Initial acceptance stage: Microsoft accepts the message during the SMTP transaction, so your LISTSERV receives the standard 250 OK response. 

During the asynchronous deep filtration stage: After accepting the message, Microsoft conducts additional checks, including advanced spam and content filtering, reputation scoring, rate limiting, and DMARC/SPF/DKIM validation, along with other policy‑based evaluations. 

If any of these deeper checks fail, even slightly, Microsoft may still generate a Delivery Status Notification (DSN) and send it back to the MAIL FROM/bounce address used by LISTSERV. LISTSERV may then interpret this DSN as a bounce. 

So, when the subscriber still receives the message, the message may have already been delivered to the user’s inbox (or Junk folder) by the time the deeper filtering occurs. Microsoft does not always retract it. As a result, the user sees the message normally, but LISTSERV receives the DSN afterward and treats the address as problematic. 

For LISTSERV, the system may apply a more sophisticated bounce‑handling process. For example: 

Permanent bounces (5xx errors) > the address is removed immediately. 

Temporary bounces or policy/reputation‑related bounces (4xx errors or certain 5xx policy errors) > after the number of bounces reaches a configurable threshold (typically several consecutive occurrences or a percentage‑based threshold), LISTSERV automatically deletes or suppresses the address to protect your sender reputation. 

This may explain why some addresses exceed the “bounce threshold” and disappear from your lists even though the user still received the email. 

Please note that this summary is based on my own findings and may not fully address your concerns. To investigate the issue in this case, I recommend reaching out to tech community forums for a deeper technical dive or to connect with individuals who may have experience and expertise in this specific customization. Some features may behave differently or be restricted depending on the specific environment and configuration. There are many experienced developers and Microsoft specialists active there who can help troubleshoot and guide you through the process.   

Apologies for redirecting you to the related development team support, as the moderators in this community have limited resources to check the backend information, and to get the fast and better assistance we requested for it.   

I hope this helps. 

Please feel free to correct me if I misunderstood your request. If you have any additional concerns, feel free to comment below. I would be more than happy to assist. 

Note: Please follow the steps in [our documentation] to enable e-mail notifications if you want to receive the related email notification for this thread. 

Hotmail/Live recipients are almost certainly returning intermittent soft bounces or policy/reputation-related SMTP errors, and LISTSERV is interpreting those as failures and automatically removing the addresses.

Key points in the mail flow and why this happens:

1. How delivery and bounces work
   • When LISTSERV sends to Hotmail/Live, the receiving server (Outlook.com infrastructure) returns an SMTP status:
     • 2xx = accepted (Delivered)
     • 4xx = temporary failure (soft bounce)
     • 5xx = permanent failure (hard bounce)
   • Some providers also generate later DSNs (bounce messages) even after initially accepting the message.
   • LISTSERV typically has rules like: after N bounces (or certain error codes), automatically delete or suspend the address.
2. Why users still receive the email
   • Many “bounces” are generated after the message is accepted:
     • The SMTP transaction returns 2xx (so LISTSERV thinks it was delivered), but later Hotmail decides the message is spam, violates policy, or hits a quota and generates a DSN back to the envelope sender.
     • From LISTSERV’s perspective, it sees a bounce message referencing that recipient and increments the bounce counter, even though the user actually got the message (or at least it was accepted by Hotmail).
   • This matches the behavior described in the context where:
     • Temporary failures and infrastructure problems are treated as soft bounces.
     • Spam/policy decisions are treated as “spam bounces” or policy-related failures, not always consistently across providers.
3. Types of errors that can cause this with Microsoft consumer mailboxes Common categories that map to what Hotmail/Live might be doing:
   • Temporary/technical issues (soft bounces)
     • Recipient server overload, connection timeouts, or message-expired conditions. These are transient and may resolve on their own, but LISTSERV may still count them as bounces.
   • Reputation or blocklist issues
     • If Hotmail/Live considers the LISTSERV IP or domain low reputation or on a blocklist, it can:
       • Accept some messages and silently junk them.
       • Return DSNs referencing “blocked” or “listed in” spam services.
     • These map to reputation-issues or blocklisting-issues categories.
   • Policy-related issues
     • Hotmail/Live may enforce stricter policies on bulk senders (rate limits, content filters, DMARC/SPF/DKIM alignment). Failures here show up as:
       • dmarc-issues
       • policy-related
       • spam bounce (provider-specific wording)
   • Message-expired / connection errors
     • If Hotmail delays or throttles delivery, the sending system may eventually give up and mark the message as expired or connection-error, even if some attempts succeeded.
4. Why this started “recently”
   • Large providers (including Outlook.com/Hotmail/Live) regularly tighten spam and bulk-mail policies. Changes can include:
     • Stricter DMARC/SPF/DKIM checks.
     • Lower tolerance for high bounce rates or low engagement.
     • New rate limits or content-based spam rules.
   • If LISTSERV’s bounce-handling thresholds are unchanged, but Hotmail has become more aggressive, more Hotmail/Live addresses will cross LISTSERV’s bounce threshold and be removed.
5. How LISTSERV’s automatic suppression interacts with this
   • Systems that protect sender reputation often:
     • Track hard bounces (invalid-mailbox, invalid-domain, inactive-mailbox) and permanently suppress those addresses.
     • Track repeated soft bounces (temporary failures, full-mailbox, policy-related, reputation-issues). After several consecutive failures, they add the address to a suppression list.
   • This is similar to the behavior described where:
     • Hard-bounced addresses are stored and blocked.
     • After multiple sequential soft bounces, addresses are also suppressed.
   • If Hotmail/Live is intermittently returning soft bounces or policy/reputation errors, LISTSERV will eventually treat those addresses as bad and remove them.
6. What to check and adjust At a high level, the resolution usually involves:
   • Reviewing actual SMTP/DSN error texts for Hotmail/Live addresses:
     • Identify whether errors are:
       • Hard bounces (invalid mailbox/domain).
       • Reputation/blocklist-related.
       • Policy-related (DMARC/SPF/DKIM, rate limits, spam content).
       • Temporary infrastructure issues (connection-error, recipient-server-error, message-expired).
   • Ensuring proper authentication and alignment:
     • SPF, DKIM, and DMARC correctly configured and aligned with the LISTSERV sending domain.
   • Tuning LISTSERV bounce thresholds:
     • Increase the number of soft bounces allowed before removal, especially for large providers like Hotmail/Live where transient or policy bounces are common.
     • Differentiate between hard bounces (remove immediately) and soft/policy/reputation bounces (delay removal, maybe flag for review).
   • Monitoring bounce categories and rates:
     • Track bounce rate specifically for Hotmail/Live domains.
     • If reputation-issues or blocklisting-issues are significant, work with the mail host or Microsoft support to address reputation and blocklist status.

This combination—Hotmail/Live generating soft/policy/reputation bounces and LISTSERV’s automatic suppression logic—explains why addresses that appear to work are later removed as “erroneous.”

References:

• Email bounce categories
• Fix a high email bounce rate
• How Customer Insights - Journeys uses suppression lists for email deliverability
• Understand sender reputation in Azure Communication Services email
• Sent email in Outlook.com comes back "delivery failed"