{"Autodiscover blocked a potentially insecure redirection to https://autodiscover-s.outlook.com/autodiscover/autodiscover.xml. To allow Autodiscover to follow the redirection, use the AutodiscoverUrl(string, AutodiscoverRedirectionUrlValidationCallback) overload."}

Error Message : - 

{"Autodiscover blocked a potentially insecure redirection to https://autodiscover-s.outlook.com/autodiscover/autodiscover.xml. To allow Autodiscover to follow the redirection, use the AutodiscoverUrl(string, AutodiscoverRedirectionUrlValidationCallback) overload."}

Solution :

Comment Below line 
 // service.AutodiscoverUrl(MailUser);

then add below code
service.AutodiscoverUrl(MailUser, (a) =>
                {
                    return true;
                });