Ransomware attacks on on-premises Exchange Servers are quite common as they store sensitive and confidential information and business data. The attackers often exploit vulnerabilities to gain access to the organization's network and steal or encrypt data for ransom.
Even if attackers do not succeed in encrypting the data, they may leave a backdoor or cause other damage that can leave your server unusable or cause permanent data loss. That's why it's important to have a disaster recovery plan or recovery strategy ready to deal with such critical situations.