Exchange Online Flagged Gmail Emails as Spam! Now Fixed
People were facing delayed responses and sometimes no responses at all while using Gmail IDs to send emails to the Exchange Online accounts between 29 April to May 1st. This was because of a simple reason that Exchange Online flagged Gmail emails as spam.
Upon diagnosis, it was found that a hastily implemented ML filter was causing this to happen.
Now that the issue is fixed and the investigation is complete, we can make use of the 20/20 hindsight to go over what exactly happened and what can be done to safeguard communication channels against such issues. Before that, a bit of background on the issue itself.
Timeline of the Exchange Online Flags Gmail Emails as Spam Issue?
Exchange Online, being a cloud-based email and calendar management service, is used by millions of organizations. Which means Microsoft makes regular updates to this critical infrastructure.
It was during one such update that rolled out around the last week of April 2025 that put it in conflict with Gmail, another email behemoth.
As early as 25th April M365 users (both free and paid) began to notice that any new message that came specifically from a Gmail account was not being shown in the inbox. Rather, Exchange Online began flagging those messages as spam and diverting them to Junk folders or a quarantined state.
However, it was interesting to note that there was no problem in communication when roles were opposite i.e., Exchange was the sender and Gmail was the recipient.
Soon, Microsoft took notice of the situation and began tracking it under incident number EX1064599, as seen on the Microsoft 365 admin center.
This problem was service wide meaning that both personal account holder as well as those who has a business side subscription faced the problem. Interestingly no complains came from the on premise self maintained Exchange Server users. However there were no checks made to confirm if any Exchange server is sending spam mails or not.
What Impact Did Exchange Online Flagging Gmail Emails Have On Users?
The time spread between the first user reports and eventual issue resolution was quite large (almost a week) by today’s ssatndard. Although the issue was not that high up on the severity chart as the emails did arrive. Plus they came to the right recipient. A simple click on the move to inbox button was all it took to resolve the in convenience.
Which was not too big of an issue for one or two odd mails. However, users who regularly dealt with vast volume of external emails (most of which came from free Gmail accounts) had a hard time. Hybrid users had to figure out if it was their Outlook which was not connecting to Exchange Server or the Exchange Online service that was misbehaving.
As users had to manually sort out the genuine emails from actual junk this was a time consuming low productivity work. Not to mention the ever-present possibility of missing a genuine mail and re positioning an actual junk mail.
Emails in the junk folder often go unnoticed, and as this issue emerged out of nowhere, there was not much official information.
The initial diagnosis attributed this error as a result of a faulty machine learning filter.
How did Users Do to Manage to Maintain Communication Lines During the Service Misbehavior?
Email communication being such a vital part of daya to business communication many uses took matters into their own hand.
To assist users Microsoft announced a set of mesures which prompted administrators to create custom allow rules.
Where they could use the Tenant Allow/Block List or Transport Rules in Exchange Online and set the SCL to -1 for messages coming from Gmail domains.
This served as an effective remedy while Microsoft reverted the Exchange Online server back to a previous version of the ML model.
Lessons Learned and Next Steps
This was not the first time that such an issue affected a users ability to see incoming email. Rather many saw it as a part of a broader trend of ML-related errors in Exchange Online.
Not even a week had passed where a similar issue flagged Adobe emails as spam. As many may already considered Adobe emails as spam so the issue did not cause much of an issue. There were other incidents in March of this year. Anectodal user reports suggested that Exchange Online service was incorrectly quarantining mails.
Seeing how such issues are occuring more and more frequently its apt time for organizations to ensure email data safety at thier end.
The best possible way to do that is to use SysTools Office 365 Backup Tool. The best part about this tool it that it recreates the entire Exchange Online folder structure offline. So even if an genuine email finds it way into the spam folder this tool still downloads it and keeps it safe on your workstation.
As the email exist outside the actual Exchange Online junk email folder it is no longer subject to the strict 30 day expiry period. So admins should get a coy of this tool right now ans keep their organizational data safe.
Tabular Summary of the Exchange Online Gmail Spam Flagging Issue
The following table summarizes the incident details:
| Aspect | Details |
| Start Date | April 25, 2025, at 09:24 UTC |
| Resolution Date | May 1, 2025, at 16:31 UTC |
| Incident Code | EX1064599 |
| Cause | A faulty ML model in EOP, mistaking Gmail emails for spam due to similarity |
| Impact | Gmail emails are moved to the junk folder or quarantined, users face inconsistent delivery |
| Fix | Reverted to the previous ML model, confirmed via telemetry |
| Workarounds | Custom allow rules, set SCL to -1 for Gmail domains |
| Ongoing Actions | Investigating ML improvements to reduce false positives |
| Related Incidents | Adobe emails (recent), similar quarantining in March 2025, past issues |
Conclusion
Towards the end of April and the start of May 2025, an issue popped up where Exchange Online flagged Gmail emails as spam. Although this was a significant disruption to regular mail flow, Microsoft quickly identified the root cause and rolled it back. This incident faulty self induced error once again underlines how significant it is to have control over one’s data. So an easy way to do that is to form an email backup using the a professional utility.