Problem in receiving emails to hotmail accounts in asp.net

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VB

ASP.NET

SMTP

mail

SmtpClient

Hello,

I'am developing a website "www.synergyprosale.com" using asp.net i'am facing a serious strange problem, which is the following:
I'm using smtp email technique in sending email from the website, but the emails are not delivering to hotmail accounts, while they are receiving successfully to other types of accounts as gmail, yahoo,..etc.
The code below was working properly, but without any changes in the code neither in the configuration it stopped sending to hotmail accounts.
Knowing that the user is not getting any error or warning.
Hope you can help me in fixing this issue.

Below you can find my code:

Public Function EmailSending(ByVal mailto_ As String, ByVal mailFrom As String, ByVal subject As String, ByVal body As String) As Boolean
            Dim basicAuthenticationInfo As New System.Net.NetworkCredential("***@synergyprosales.com", "******")
            Dim mail_from As System.Net.Mail.MailAddress = New System.Net.Mail.MailAddress(mailFrom, "Synergy Prosales Company")
            Dim mail_to As System.Net.Mail.MailAddress = New System.Net.Mail.MailAddress(mailto_)
            Dim message As System.Net.Mail.MailMessage = New System.Net.Mail.MailMessage(mail_from, mail_to)
            message.Subject = subject
            message.Body = body

            Dim smtp As New System.Net.Mail.SmtpClient("relay-hosting.secureserver.net")
            smtp.UseDefaultCredentials = false
            smtp.Credentials = basicAuthenticationInfo
            smtp.EnableSsl = false
            smtp.Host = "relay-hosting.secureserver.net"
            smtp.Port = 25
            smtp.Send(message)
            Return True
    End Function

Many thanks in advance.

Best Regards,
Dima
Senior Web Developer