または、文字列ベースのソリューションの場合:
string address = "[email protected]";
string Host;
// using Split
Host = address.Split('@')[1];
// using Split with maximum number of substrings (more explicit)
Host = address.Split(new char[] { '@' }, 2)[1];
// using Substring/IndexOf
Host = address.Substring(address.IndexOf('@') + 1);