Why do I have problem sending Email to external servers

I have developed an ASP.NET webforms application. I'm trying to send a form filled to an internal account and then a copy to the email account of the user. I dont get any errors. Internal mails are sent while external emails dont get sent. I also noticed I could not send email from a some of my email accounts to external emails like gmail or yahoo, from even an email client application

Here is the line of code :

SmtpClient smtpClient = new SmtpClient("mail.domainhost.co.uk",25);
NetworkCredential credentials = new NetworkCredential("userName", "password");
smtpClient.Credentials = credentials;

MailMessage mailMessage = new MailMessage(FromAddress,ToAddress,Enquiry,Message);
mailMessage.IsBodyHtml=true;
mailMessage.Priority = MailPriority.High;
int i = 0;
try
{
smtpClient.Send(mailMessage);
i = 1;
}
catch(Exception e)
{
i = 0;
}
return i;

Can I get help with this please?

 

I am having a very similar problem. Were you ever able to resolve this?