C# SMTP enviado desde un servidor que no está en el mismo dominio que Exchange está en blanco

I've just written a piece of code for a MVC website that sends a SMTP email using the .NET SmtpClient via our Exchange server. The email it sends has a HTML body with links to images and a file that are hosted on the website.

The email is sent fine when run internally on our network, but when its run from the hosted server that's not on our domin, the email comes through but the body is blank. Does anybody have any idea why? Is is because of the linked images or file that could be a potential threat and come from a server not on the domain and so is not trusted?

Here's the code that sends the email, it uses the MailDefinition class to insert a link to a file into the body that they've requested to download:

MailDefinition md = new MailDefinition();
md.From = "test@testing.com";
md.Subject = "Test Email";
md.IsBodyHtml = true;

ListDictionary replacements = new ListDictionary();
replacements.Add("REQUESTED_LINK", @"C:\MyFile.pdf");

MailMessage email = md.CreateMailMessage(mailTo, replacements, content, new System.Web.UI.Control());

SmtpClient emailClient = new SmtpClient();
emailClient.Host = "MyExchangeServer";

emailClient.Send(email);