Mailbox name not allowed. The server response was: Relaying disallowed

I am trying to send email through asp.net application using zoho to my gmail id( something@gmail.com ) and to email ids created on zoho( something@mydomainname.com ). I am able to send emails to email ids created on zoho( something@mydomainname.com ) but unable to send emails to gmail ids( something@gmail.com ). Its giving this error " Mailbox name not allowed. The server response was: Relaying disallowed"

Following is the c# code I am using:

string MailServer = mx.zoho.com;

string AdminEmail = System.Web.Configuration.WebConfigurationManager.AppSettings["MailServerEmail"];

string AdminEmailPassword = System.Web.Configuration.WebConfigurationManager.AppSettings["MailServerPassword"];

string[] emailAddresses = ToEmail.Split(',');

System.Net.Mail.MailMessage message = new System.Net.Mail.MailMessage();

message.From = new MailAddress(FromEmail, From, System.Text.UnicodeEncoding.UTF8);

foreach (string emailAddress in emailAddresses)

message.To.Add(new MailAddress(emailAddress));

message.SubjectEncoding = System.Text.UnicodeEncoding.UTF8;

message.Subject = Subject;

message.Body = Body;

message.BodyEncoding = System.Text.UnicodeEncoding.UTF8;

message.IsBodyHtml = true;

SmtpClient client = new SmtpClient();

client.Host = MailServer;

client.Port = 25;

client.Credentials =

new System.Net.NetworkCredential(AdminEmail, AdminEmailPassword);

client.Send(message);