Something like this:
using System;
public class Program
{
public static void Main()
{
string str = @"Content-Type: text/xml; charset=utf-8
Content-Transfer-Encoding: binary
Content-ID: <0.8788fd1f238c5bd025268ecbf502e58726ca092a8ce8fe81@apache.org>
Here starts the real stuff, blah, blah, blaaaaaaaaaaah.";
int pos = str.IndexOf("@apache.org>") + "@apache.org>".Length;
str = str.Substring(pos);
Console.WriteLine(str);
}
}