Update: July 17th
We wanted to provide you an update on this issue.
The issue has been resolved and new updates will be available through Windows Update for all operating systems by end of day July 17th (PDT). The Exchange Team recommends that customers use Windows Update or update the catalogs on their own SUS servers to ensure the latest version of the update is available for installation on your Exchange Servers. Doing so will avoid any possible disruption to the MSExchangeTransport service which may have been impacted by the July 10th update.
Note: There will be a unique KB number associated for each Windows operating system. Applying the latest updates from Windows Update will ensure you have the correct update for your operating system regardless of updates previously applied.
After installing the July Updates for Windows on some Exchange CAS servers, the servers stopped accepting SMTP requests after a few hours. A reboot would fix the problem for a few hours and then it would happen again.
For now, remove KB4338818 and restart. It should fix the problem. Be sure to block or hide the update until they fix it.
Other issues with this update.