Microsoft Email Filtering Impact on SignNow Invite Delivery

Update timeline

1. investigating Feb 27, 2026, 04:45 PM UTC

   Some users may experience issues with SMS invite delivery. Additionally, email invites may be routed to spam folders. Our team is actively investigating the issue and will provide updates as they become available.

2. investigating Feb 27, 2026, 05:56 PM UTC

   Issue with sms invites is now being fixed. Email invites still might be routed to spam folders. Our team is actively investigating the issue and will provide updates as they become available.

3. identified Feb 27, 2026, 06:14 PM UTC

   The issue has been identified and a fix is being implemented.

4. identified Feb 28, 2026, 05:29 PM UTC

   We are still investigating intermittent email delivery issues affecting recipients on Microsoft-hosted domains (including Outlook.com, Hotmail.com, Live.com, and Microsoft 365/Exchange Online environments). Some emails sent from SignNow to Microsoft-hosted mailboxes are being accepted by Microsoft servers but may not reach the recipient’s inbox. In certain cases, messages may be silently filtered or quarantined.

5. monitoring Feb 28, 2026, 05:59 PM UTC

   This incident was caused by aggressive spam filtering behavior within Microsoft-hosted email environments (Outlook, Hotmail, Live, Microsoft 365). SignNow systems were functioning correctly and emails were successfully accepted by Microsoft servers (SMTP 250 OK). However, Microsoft’s filtering layer intermittently flagged and quarantined certain messages before reaching recipient inboxes. Microsoft has acknowledged the filtering behavior and applied backend whitelisting for signnow.com as an interim mitigation while tuning their detection models. The issue is currently mitigated and under monitoring.

6. monitoring Mar 01, 2026, 12:21 PM UTC

   We are currently awaiting confirmation from Microsoft that the applied fix has been successful.

7. resolved Mar 01, 2026, 09:55 PM UTC

   Between February 26 and February 28, Microsoft filtering behavior caused intermittent delivery issues for email invites sent to Microsoft-hosted domains (Outlook, Hotmail, Live, Microsoft 365). SignNow systems were operating normally, and messages were successfully transmitted and accepted by Microsoft servers. The disruption occurred within Microsoft’s filtering layer due to aggressive spam detection behavior. Microsoft has identified the issue and is applying backend mitigation (whitelisting) while continuing to tune their detection models. While awaiting final confirmation from Microsoft, we are proactively re-sending affected unsigned invites to impacted recipients to ensure successful delivery. We are continuing to monitor delivery behavior closely and will provide updates if needed. ________ Microsoft confirmed that fix was aplied globally and invites has to be delivered with no further issues. SignNow team will resend all unsgned invites shortly. ______ SignNow team has resent all affected emails invites.

8. postmortem Mar 05, 2026, 09:46 AM UTC

   ## **Summary** Between **February 23 and February 28, 2026**, changes in Microsoft’s email filtering behavior intermittently affected delivery of some SignNow invite emails sent to Microsoft-hosted email services, including Outlook, Hotmail, Live, and Microsoft 365. SignNow systems were operating normally during this period. Email messages were successfully transmitted from SignNow infrastructure and accepted by Microsoft servers. However, Microsoft’s filtering systems intermittently prevented some of these messages from reaching recipient inboxes. Microsoft later confirmed aggressive spam filtering behavior affecting legitimate email traffic from certain senders and applied mitigation while tuning their filtering models. After confirmation from Microsoft that mitigation had been applied, SignNow proactively **re-sent affected unsigned invites** and notified impacted document owners. Email delivery behavior is currently stable and continues to be monitored. ## **Timeline** **February 23, 2026**Early signals of email delivery issues begin to appear related to Microsoft-hosted email domains. **February 26, 2026**SignNow begins receiving customer reports that some invite emails sent to Microsoft domains are not reaching recipient inboxes. **February 27, 2026**Engineering investigation confirms that emails are successfully transmitted from SignNow infrastructure and accepted by Microsoft servers. SignNow escalates the issue to Microsoft and begins collaborating with affected customers to collect delivery traces. **February 28, 2026**Microsoft confirms aggressive spam filtering behavior affecting legitimate emails and applies temporary mitigation. **March 1, 2026**Microsoft confirms mitigation has been applied broadly. **March 2, 2026**SignNow automatically re-sends affected unsigned invites to ensure recipients receive the documents. **March 4, 2026**Targeted communication is sent to document owners whose invites may have been affected. ## **Root Cause** The incident was caused by **Microsoft spam filtering behavior driven by machine-learning detection models** that intermittently classified some legitimate emails as suspicious. Although SignNow messages were successfully transmitted and accepted by Microsoft mail servers, some emails were filtered before reaching recipient inboxes. A sender header configuration detail may have increased sensitivity to Microsoft filtering behavior for certain messages. This configuration has since been reviewed and adjusted. ## **Impact** During the affected timeframe: * Some invite emails sent to Microsoft-hosted domains experienced delivery delays or filtering. * The issue was **intermittent** and did not affect all messages. * A subset of senders who frequently send documents to Microsoft-hosted domains experienced higher likelihood of impact. SignNow received a limited number of customer reports related to missing invite notifications. Each report represented multiple recipients, meaning the number of impacted recipients was higher than the number of reported cases. ## **Mitigation and Actions Taken** During the investigation, SignNow took the following steps: * Investigated email delivery logs and confirmed successful transmission from SignNow infrastructure * Adjusted email header configuration to reduce filtering sensitivity * Escalated the issue to Microsoft support * Collaborated with affected customers to gather delivery traces * Monitored delivery behavior across Microsoft-hosted domains * Automatically re-sent affected unsigned invites * Proactively notified impacted document owners ## **Current Status** Based on Microsoft confirmation and subsequent delivery validation, the issue is considered **resolved**. SignNow continues to monitor delivery performance to ensure consistent email delivery to Microsoft-hosted domains.