I have no idea for free backup service but I have got the below list from a forum and it might help you out. Here’s the best backup software for Windows Server
-Acronis Cyber Backup
-AOMEI Backupper Server
-Paragon Backup & Recovery Business
-NovaBACKUP for Servers
-Windows Server Backup
Here are some links to help you out
Use Windows Server Backup to back up Exchange Server
https://learn.microsoft.com/en-us/exchange/high-availability/disaster-recovery/backup-with-windows-server-backup?view=exchserver-2019
Exchange Server: Use Windows Server Backup to back up and restore Exchange data
https://learn.microsoft.com/en-us/exchange/high-availability/disaster-recovery/windows-server-backup?view=exchserver-2019
Hope this resolves your Query!!
--------
--If the reply is helpful, please Upvote and Accept it as an ant