Cannot send emails from C# code to any other domain but mine

Hi,
it seems I can send emails from my C# code only to email addresses hosted on my own domain, but not to other domains such as gmail or hotmail. Although I can send emails to any address from the SmarterMail web client on m05.internetmailserver.net and from Windows Live Mail (running on my local machine).
I already went through all the forum entries and couldn't find a solution.
My DNS configuration for mydomain.com that can be found at: https://cp.Winhost.com/domains/dns.aspx is:

A Record for mail domain:

Domain Name: mail.mydomain.com
IP Address: AAA.BBB.CCC.DDD
TTL: 3600

When this IP address is copied-and-pasted in a web browser, it resolves to the web page login of SmarterMail on m05.internetmailserver.net

There are two more A record for "mydomain.com" and "ftp.mydomain.com" which resolve to another IP address.


MX Record:

Domain Name: mail.mydomain.com
Mail Server: m05.internetmailserver.net
Priority: 10
TTL: 3600

No other MX Records exist.





My C# test code (running from my local machine) is:


// GIVEN
// Works fine with "mydomain.com" email addresses such as "[email protected]"
// Doesn't work with email addresses not hosted on "mydomain.com" such as gmail.com, hotmail.com, ...
// Of course "mydomain.com" here is just a placeholder for the actual domain name which is embedded in my real code.
var recipientEmailAddress = "[email protected]"; // In my actual code I use an existing email address, the one showed here is just a placeholder.

var msg = new MailMessage
{
BodyEncoding = Encoding.Default,
IsBodyHtml = true,
Subject = emailSubject,
Body = emailHtmlBody,
From = new MailAddress("[email protected]"),
};
msg.To.Add(new MailAddress(recipientEmailAddress));

var smtpClient = new SmtpClient("mail.mydomain.com") // nothing changes if I use the MX record to which mail.mydomain.com points to: m05.internetmailserver.net
{
Credentials = new NetworkCredential("[email protected]", "postmaster password"),
DeliveryMethod = SmtpDeliveryMethod.Network,
Port = 587, // also tried 25 and 465, but they don't seem to work
Timeout = 60000,
UseDefaultCredentials = false,
};


// WHEN-THEN
smtpClient.Send(msg);




Hangs for a few seconds then throws the exception:

System.Net.Mail.SmtpFailedRecipientException: Mailbox not available. The server answered: <[email protected]> No such user here




What am I doing wrong?

Thank you for your help.

 

Hi,
thank you for your reply. I followed your example in the Knowledge Base when I wrote the code I posted here.
Isn't SMTP authentication taken care of by the NetworkCredential instance where I pass the postmaster username and password?
Thank you.

 

I'm running a .Net 4.5 console application which will become a windows service on Azure, so I couldn't set the full trust level as mentioned in your KB. I found another way and now it's full trust.
The problem remains, so I'm sending you a private message.
Thank you for your time and effort.

 

Hi I am having the same problem on www.appsbydaniel.com, what was the resolution of this please?

 

Assuming you've tried SMTP authentication and Full Trust as discussed earlier, you might want to open a support ticket and we"ll take a look.

 

Hi Michael, thanks, but as it happens it was a coding error on my part, so it is resolved