What is the meaning of A potentially dangerous Request.Form value was detected from the client error message

Question

I was trying to send an email via my web project using Free text box control.It is an text editor . but I\'ve got this error.

A potentially dangerous Request.For

Answer 1

Please see the answer on this question:

ValidateRequest="false" doesn't work in Asp.Net 4

This is a change in .net 4.0.