Winhost to Hotmail - System.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable

I am having issues sending a confirmation email to user after registration. I've tried 2 Hotmail email and I got Mailbox unavailable error. It show "System.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable. The server response was: <[email protected]> No such user here". I read some threads, but I couldn't get an answer.
The first time, I used my own Hotmail email, and the second time I used my brother's email. I got the same error message.
I successfully logged into my account on mail.mydomain.com with the username [email protected], and from there I can send an email to the Hotmail account. But from my Winhost hosted website, It does not work. Can anyone provide help?

Here is my web.config

<system.net>
<mailSettings>
<smtpfrom="[email protected]">
<networkhost="mail.mydomainname.com"password="mypa$$w0rd"
userName="[email protected]"defaultCredentials="true" />
</smtp>
</mailSettings>
</system.net>
==================
Here is the VB code behind

Dim TextMsg AsString = "Thanks for registering..."
Dim SenderEmail AsString = Trim(txtUserEmail.Text)
Dim SenderName AsString = Trim(txtName.Text)
Dim MailMsg AsNewMailMessage()
MailMsg.BodyEncoding = Encoding.Default
MailMsg.Subject = "Welcome to..."
MailMsg.Body = TextMsg
MailMsg.Priority = MailPriority.High
MailMsg.IsBodyHtml = True
MailMsg.From = NewMailAddress("[email protected]", "Domain Moto")
MailMsg.To.Add(NewMailAddress(SenderEmail, SenderName))

'SmtpClient to send the mail message
Dim SmtpMail AsNewSmtpClient
SmtpMail.Send(MailMsg)
================

After I enable debug=true in the web.config, it pointed out the line SmtpMail.Send(MailMsg). I don't know what it wrong. I can see in the database that the user has been added, but still I can't send a confirmation email to Hotmail account.

 

I opened a ticket, but the support I got was not satisfactory. Anybody has an idea why this problem is happening?

My code can send, and does send an email, but unfortunately the email to external network such as Hotmail, webmails, yahoo does not work. A copy of the email is correctly send into my domain's email account on winhost. I review my web.config file, and here is what I actually have.

<system.net>
<mailSettings>
<smtp from="[email protected]">
<network host="mail.mydomain.com" password="myPas$w0rd" port="25"
userName="[email protected]" defaultCredentials="true" />
</smtp>
</mailSettings>
</system.net>

Here is my code behind

Dim SenderEmail As String = Trim(txtEmail.Text).ToString
Dim SenderName As String = Trim(txtName.Text).ToString
Dim MailMsg As New MailMessage()
MailMsg.BodyEncoding = Encoding.Default
MailMsg.Subject = MsgSubject
MailMsg.Body = theOrder
MailMsg.Priority = MailPriority.High
MailMsg.IsBodyHtml = True
MailMsg.From = New MailAddress("[email protected]", "Support at mydomain.com")
MailMsg.To.Add(New MailAddress("[email protected]", "My Company Name"))
MailMsg.CC.Add(New MailAddress(SenderEmail, SenderName))
'Smtpclient to send the mail message
Dim SmtpMail As New SmtpClient
SmtpMail.Send(MailMsg)

Any thoughts?