Mohammed,

The release you're currently using is 24 minor releases behind the final release of SmarterMail 16 (16.3.6989).  There were a lot of bug fixes in between those two releases, so the first thing I would recommend trying is to upgrade to that release and see how things behave.

Thanks for the response.  How do I get the 24 minor releases?

issah

https://www.smartertools.com/smartermail/downloads#/pills-previousbuilds you have to login to your SmarterTools account to access the older v16 builds, but you can get them at the URL above.

Mohammed, also, you do not need to upgrade to each minor update. You can perform a minor update to the latest release of SmarterMail v16 using these links here: 

If you require further assistance with this I recommend getting a support ticket submitted at your earliest convenience so we can help narrow down the root cause. Have a great day!