vb.net how to detect if an email is undeliverable in a win form program - vb.net

Question

I have the following code:
Public Function VerstuurMail(ByVal strFrom As String, ByVal strTo As String, ByVal strSubject As String, ByVal strBody As String, ByVal strMailSMTP As String, ByVal MailUser As String, ByVal MailPassword As String, ByVal MailPort As Integer, Optional ByVal AttachmentFiles As String = "") As String
Try
'create the mail message
Dim mail As New MailMessage()
Dim basicCredential As New NetworkCredential(MailUser, MailPassword)
'set the addresses
mail.From = New MailAddress(strFrom)
mail.To.Add(strTo)
'set the content
mail.Subject = strSubject
If File.Exists(strBody) = True Then
Dim objReader As New System.IO.StreamReader(strBody, System.Text.Encoding.GetEncoding(1252))
mail.Body = objReader.ReadToEnd
objReader.Close()
End If
mail.IsBodyHtml = False
'send the message
Dim smtp As New SmtpClient(strMailSMTP)
smtp.DeliveryMethod = SmtpDeliveryMethod.Network
smtp.EnableSsl = True
'smtp.UseDefaultCredentials = True
smtp.Credentials = basicCredential
smtp.Port = MailPort
Dim AttachmentFile As String() = AttachmentFiles.Split("*")
For Each bestand In AttachmentFile
If System.IO.File.Exists(bestand) Then
mail.Attachments.Add(New Attachment(bestand))
Else
Call MessageBox.Show("File can't be found")
End If
Next
'Dim userState As Object = mail
'smtp.SendAsync(mail, userState)
'AddHandler smtp.SendCompleted, AddressOf SendCompletedCallback
smtp.Send(mail)
mailSent = True
smtp.Dispose()
Catch ex As Exception
mailSent = False
Call MessageBox.Show(ex.Message & vbCrLf & "Didn't sent to: " & strTo & vbCrLf & " with extra error message:" & vbCrLf & ex.ToString)
Finally
End Try
Return mailSent
End Function
This function is used in a program which reads a text file, with the parameters on one line, and is called as many lines there are. (in a loop)
This is working fine.
Now when the text file has a wrong email adress the function doesn't trow a error it just sent the email to nobody.
example: sent an mail to joe#gmail.com works, send an email to joe#hmail.com doesn't sent but doesn't give an error either.
I have googled but the examples said that I should use 'smtp.SendAsync(mail, userState)'
But then the program doesn't follow the loop anymore and no mails are being sent. I can't use the debugger and step through the code. It just jumps from one place to the other.
This is the other function:
Private Sub SendCompletedCallback(ByVal sender As Object, ByVal e As AsyncCompletedEventArgs)
Dim mail As MailMessage = CType(e.UserState, MailMessage)
'write out the subject
Dim subject As String = mail.Subject
'If e.Cancelled Then
' Call MessageBox.Show("Send canceled: " & Now & " token " & subject)
' MailLog &= "Send canceled" & vbCrLf
'End If
If Not e.Error Is Nothing Then
Call MessageBox.Show("Foutmelding op: " & Now & " onderwerp " & subject & " met error: " & e.Error.ToString())
MailLog = MailLog & "Niet verstuurd met als fout melding: " & e.Error.ToString() & vbCrLf
Else
'Call MessageBox.Show("Message sent at: " & Now)
MailLog = MailLog & "Bericht verstuurd op: " & Now & vbCrLf
End If
mailSent = True
End Sub
Thanks in advance. I hope somebody can put me in the right direction.
Brian

Answer 1

This is a data issue, not an issue with the actual mechanism to send email. The best you can do is to use a regular expression to make sure the email address is valid per the rules of RFC 2822, like this:
string email = txtemail.Text;
Regex regex = new Regex(#"^([\w\.\-]+)#([\w\-]+)((\.(\w){2,3})+)$");
Match match = regex.Match(email);
if (match.Success)
{
Response.Write(email + " is valid.");
}
else
{
Response.Write(email + " is invalid.");
}
Unfortunately for you, joe#hmail.com is a valid email address by the above logic, but is is not the intended joe#gmail.com address so it ends up in the wrong place. When it is discovered that it is the wrong email address, then changing the data to the right value is the only correct course of action.
Note: Generally, you verify someone's email address when they register for a website, thus the system "knows" the email address is legitimate/correct because they successfully received an email and entered a verification code or clicked on a link that verified with the website that they did get the email. This solves most data issues (misspellings, incorrectly entered values, etc.).

Answer 2

After two full days of testing and searching I have found that when using:
Dim userState As Object = mail
smtp.SendAsync(mail, userState)
AddHandler smtp.SendCompleted, AddressOf SendCompletedCallback
you should not close the smtpClient with smtp.Dispose()
The callback function kept getting the cancel error.
Also I used the backgroundworker for sending the many mails but the async is, in a way, a backgroundworker. So I had two backgroundworkers and that didn't work at all.
It took me two days.....
Brian