This indicates an attack attempt to exploit an Arbitrary File Upload vulnerability in Telerik UI for ASP.NET AJAX components.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application when handling a crafted SMTP request. An unauthenticated remote attacker may be able to exploit this to upload an arbitrary file, leading to a possible code execution condition.
Telerik UI for ASP.NET AJAX between Q1 2011 (2011.1.315) and R3 2016 SP2 (2016.3.1027)
Telerik UI for ASP.NET AJAX between R1 2017 (2017.1.118) and R2 2017 SP1 (2017.2.621)
Telerik UI for ASP.NET AJAX between R2 2017 SP2 (2017.2.711) and R3 2019 (2019.3.917)
Telerik UI for ASP.NET AJAX R3 2019 SP1 (2019.3.1023) and later
System Compromise: Remote attackers can execute arbitrary code on vulnerable systems.
Apply the most recent upgrade or patch from the vendor.