Hi Valentin,
This isn't an issue that I've personally seen or one that I've heard reported, and I'm unfortunately not sure what might be causing it. This might require a review by the Support Department.
Before that, however, would you please try upgrading your server to the latest version? 16.3.6474 was released on Sep 22, and since then, we've had many updates. While the release notes don't show a fix specifically for this, it
could be resolved with an upgrade to 16.3.6579. Here's how to perform a
Minor Upgrade of SmarterMail.
If the issue persists, I'd be happy to help you contact Support for further troubleshooting.
Andrea Free
SmarterTools Inc.
877-357-6278
www.smartertools.com