CRITICAL: Build 9540/9543 Protocol Failure - Recurring Outlook Hangs on Large Accounts/Complex Folder Structures

Problem reported by Hasan Turkmen - 2/19/2026 at 1:16 AM

Resolved

Hi,

I am writing to report a critical and recurring failure in IMAP connectivity following the Build 9540 and 9543 updates. Our investigation shows that your 'security hardening' and 'RFC compliance' changes have severely broken Outlook integration for power users.

We have identified two specific triggers for this failure:

Mailbox Size: Accounts with 5GB+ data are consistently failing.

Folder Complexity: Even small accounts (e.g., 300MB) are hanging if they utilize a large number of IMAP folders.

The most frustrating part is that re-creating the Outlook profile is NOT a permanent solution. Even after a clean profile setup and full resync, the connection stays stable for only 3 to 4 hours before it falls back into the same 'Send/Receive' hang. The client simply stops receiving responses from the server and stays in a permanent 'Not Responding' state during synchronization.

It is unacceptable that a paid enterprise solution like SmarterMail is currently outperformed by free, open-source alternatives like Dovecot, which handles these exact same accounts and folder structures without any protocol exceptions or hangs.

We cannot continue to force our customers to perform manual OST surgery every 3 hours. We need an immediate fix for the IMAP command-handling logic that is causing these sessions to freeze.

(Customers outlook versions: 2019-2025 Turkish. Outlook Classic and Outlook new mixed)
(This issue isn't just happening with one customer; it's happening with around 30-40 customers)
Outlook version change not fixed this issue.

3 Replies

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George Replied

2/19/2026 at 8:27 AM

According to my findings, the problem affects MS Outlook users who have localized folder names in their profiles. All those who had synchronization problems had localized names. Those who had English names in their profiles did not report any problems.

We fixed most of them and converted the folder names to English. Synchronization now works without any problems. However, we do not want to go through this nightmare again. I will carefully consider the next upgrade. I will first upgrade the test server and verify that it works as expected.

Derek Curtis Replied

2/19/2026 at 9:58 AM

Employee Post Marked As Resolution

As mentioned in another thread where George posted, after some troubleshooting of our own, we were able to find a very specific scenario affecting IMAP accounts in Outlook, and folder names with some specific non-ASCII characters. Incidentally, eM Client and Thunderbird appear to work just fine, so it appears it's limited to Outlook. We've seen very few tickets on this, some from people here in the Community, but it's an issue so we'll get a fix out for it today.

Derek Curtis CCO SmarterTools Inc. www.smartertools.com

Hasan Turkmen Replied

2/19/2026 at 3:16 PM

Thanks Derek, We are eagerly awaiting the update together with our customers.

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