I've recently been trying to send emails using ASP.NET 2.0 on a web server which uses MailEnable. MailEnable had relaying locked down to avoid opening the server up to spammers, but I wasn't able to authenticate with MailEnable to relay emails to external domains.
I tested my code was working by temporarily checking the "Allow relay for local sender addresses" checkbox in the SMTP properties on MailEnable and my email was relayed successfully. However as soon as I uncheck this option I get this error message in the SMTP W3C logs:
I eventually got it working. I changed the authentication type from "MailEnable integrated authentication" to "Authenticate against the following username/password." then I supplied a username and password to MailEnable.
After restarting the SMTP service in MailEnable. I was able to relay emails using the following C# ASP.NET 2.0 code snippet...
MailAddress source = new MailAddress("email@example.com", "Server");
MailAddress recipient = new MailAddress("firstname.lastname@example.org", "External");
MailMessage enquiryMail = new MailMessage(source, recipient);
enquiryMail.Subject = "Test";
enquiryMail.Body = "Test content";
enquiryMail.IsBodyHtml = true;
SmtpClient smtpServer = new SmtpClient("mail.localdomain.com");
smtpServer.Credentials = new NetworkCredential("username", "password", "localdomain.com");