James,

We are not sure exactly which update caused it. So far about 12 to 15 customers have reported this issue. Here is what we've done on our end.

• We have tried to reproduce this problem on two separate machines without success. One is running Windows 10, and other is running Windows 2012 R2. Both are patched with the latest updates from Microsoft. We've sent more than 10,000 messages within a short period and they both accepted them normally.
• The number of clients using Xeams on Windows is in thousands. The fact that only a few clients are running into this issue indicates the problem is happening due to a combination of patches on certain machines. This makes debugging a lot harder.
• Here is what's happening:
  • Every incoming email into Xeams creates a new TCP/IP connection. This connection stays open for a very small duration, typically less than a few seconds.
  • After opening a few thousand connections, the operating system blocks future connections and Xeams is no longer able to accept new emails.
  • The OS continues to block these connections even AFTER the process is terminated - meaning even if Xeams is restarted. This blockade does not end until you restart the machine. Normally, the operating system should release every resource (network connections, file handles, etc...) once a process terminates.
• We created a small program that tries to open a TCP/IP connection to any port. Stop Xeams service once the problem starts and run this program instead to recreate the issue, confirming that the OS is not releasing the connections and continues to block them. You can download this test program from here along with its source code.

Unfortunately, at this time the only recommendation we can make is to uninstall the updates that were pushed in the last few days. We will continue to work on this problem and will let you know once a solution is found.