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Hi All,
I am using Gmail setting for sending mail, i got an error like "Failure sending mail." and InnerException as "Unable to read data from the transport connection: net_io_connectionclosed."

My project frame work is .Net 4.8
Programming Language vb
OS is Windows 11

Anything need to change in Gmail account setting ?

What I have tried:

Imports System.Net
Imports System.Net.Mail
Public Class Form1
    Public Shared Function ServerCertificateValidationCallback(ByVal sender As Object, ByVal cert As System.Security.Cryptography.X509Certificates.X509Certificate, ByVal chain As System.Security.Cryptography.X509Certificates.X509Chain, ByVal sslPolicyErrors As Net.Security.SslPolicyErrors) As Boolean
        Return True
    End Function
 Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
            System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.SystemDefault Or SecurityProtocolType.Tls Or SecurityProtocolType.Tls11 Or SecurityProtocolType.Tls12
            Dim Mail As New System.Net.Mail.MailMessage
            Dim smtp As New System.Net.Mail.SmtpClient
            Mail.From = New MailAddress("", "Aravind")
            Mail.IsBodyHtml = True
            Mail.Subject = "Hai Testing Mail"
            smtp = New SmtpClient("", "587")
            smtp.EnableSsl = True
            smtp.DeliveryMethod = SmtpDeliveryMethod.Network
            smtp.UseDefaultCredentials = False
            smtp.Credentials = New System.Net.NetworkCredential("", "xxxxxxxxxxx")
            Dim userstate As Object = Mail
            System.Net.ServicePointManager.ServerCertificateValidationCallback = AddressOf ServerCertificateValidationCallback
        Catch ex As Exception
        End Try
    End Sub
End Class

Same code with another gmail account working fine,
Note: i created app password from security tab, one account working another cant, both i enabled 2 step verification.
Updated 26-Aug-22 9:50am

1 solution

Gmail uses port 465 for SSL on SMTP.

Outgoing mail (SMTP) server

Requires SSL: Yes

Requires TLS: Yes (if available)

Requires Authentication: Yes

Port for SSL: 465

Port for TLS/STARTTLS: 587

Aslso you Dim the smtp as a new smtpclient then assign it later with a new smtpcliet, one or the other will do and you can initialise the smtpclient on creation : smtp = new smtpclient(Server, Port).

Microsoft recommend NOT using smtpclient but alternatives like MailKit instead.

SmtpClient Class (System.Net.Mail) | Microsoft Docs[^]
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