Exchange 2016 Server C: Drive spaces is keep on getting reduced.

We do not use Exchange 2016 since we use cloud-based Exchange but without knowing your Exchange server configuration, this may be more of a configuration issue.  During my early years of Exchange 5.5/2000, we kept application on the C: drive and database/transaction logs on the E: drive. Never had an issue with database size except for application limitations. Transaction logs is what you need to watch out for as that can take up space, but when a full backup completes, those logs are supposed to be truncated and deleted.

But in any event, I found this old article which may help your situation:

https://www.admin-enclave.com/en/articles/exchange/296-resolved-exchange-hub-transport-mail-que-file-large-in-size.html

Again, we don't use in-house server, so this is the best that I could assist with.

Thank you both for your valuable suggestions. We will try to schedule the script share by Michelango !!!

Thanks a ton Michelangelo !!!!