无法发送邮件（VB.net）

Question

I am trying to send a mail using Dart Control. It was working well with a particular server, but since I switched to a secured server, I receive the following error message:

Protocol Exception-- Request: AUTH LOGIN Response: 530 Must issue a STARTTLS command first

Below is the code :

Dim Smtp1 As Smtp = New Smtp
Dim SMTPResult As SmtpResult
Dim Message As Dart.Mail.MailMessage = New Dart.Mail.MailMessage()

Smtp1.Session.RemoteEndPoint.Port = intPortNo
strErrLoc = "SMTP1.DnsServerTimeout"
Smtp1.DnsServerTimeout = 15 'default time out 30 seconds

strErrLoc = "Set User Name"
Smtp1.Session.Username = strUserID
Smtp1.Session.Password = strPWD

strErrLoc = "Subject and Mail TEXT"
Message.Subject = strSubject
If strHTMLEmail.Trim = "" Then
    Message.Text = strMailText
Else
    Message.Html = strHTMLEmail
End If

Smtp1.Session.Authentication = Authentication.Auto

Smtp1.Session.RemoteEndPoint.HostNameOrAddress = strServerName.Trim
Smtp1.Session.ServicePrincipleName = "SMTP/" & strServerName.Trim


SMTPResult = Smtp1.Send(Message)

Answer 1

try with enabling SSL mode,like

 Smtp1.EnableSSL = True

this works in case of Gmail ,Please Check with yours.

Answer 2

The suggestion is to use Explicit security.

The following Code implementing Explicit Security resolved the issue.

Smtp1.Session.Security.Encrypt = Encrypt.Explicit