C# sending email code suddenly stopped working

Question

i have the following code that has worked fine for months but suddently stopped working on this line:

    smtpClient.Send(msg);

this is an internal application sending emails to internal people in my company

with the following error:

Mailbox unavailable. The server response was: 5.7.1 Unable to relay for ABC@comp.com

can anyone think of a reason why this code would work fine for ages and suddently just stop work ?

        MailMessage msg = new MailMessage();
        msg.From = new MailAddress(fromEmailAddress_);
        msg.IsBodyHtml = true;

        msg.To.Add(new MailAddress(email.Trim()));

        msg.Subject = subject_;
        msg.Body = body_;
        msg.IsBodyHtml = true;
        msg.Priority = MailPriority.High;

        var smtpClient = new SmtpClient(_mailServer);
        smtpClient.UseDefaultCredentials = false;
        smtpClient.Credentials = new System.Net.NetworkCredential(_user, _pwd);

        try
        {
            smtpClient.Send(msg);
        }
        catch (Exception ex)
        {
            Console.Write(ex.Message);

        }