E-mail through earthlink SMTP server, using VB.NET, now fails

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authentication

mail

SmtpClient

For 10 years I've used same code - very similar to that which follows. All of a sudden I'm getting an error: "Mailbox Unavailable 5.5.1 Authentication required".

Using VB.net, my code:

Imports System
Imports System.IO
Imports System.Net.Mail

Public Class Form1

Private Sub btnSend_Click(sender As Object, e As EventArgs) Handles btnSend.Click

Dim strMyMailAddress As String
Dim strMyPassword As String
Dim strRecipient As String

Dim SmtpServer As New SmtpClient()

SmtpServer.Credentials = New Net.NetworkCredential(strMyMailAddress, strMyPassword)
SmtpServer.Host = "smtpauth.earthlink.net"
SmtpServer.EnableSsl = False

SmtpServer.Port = 587

Dim mail As New MailMessage()
mail = New MailMessage()
Try

mail.From = New MailAddress(strMyMailAddress, "Pick A Name", System.Text.Encoding.UTF8)
mail.To.Add(strRecipient)
mail.Subject = "Test Send"
mail.Body = "Test Send E-Mail Body Text"
mail.DeliveryNotificationOptions = DeliveryNotificationOptions.OnFailure
SmtpServer.Send(mail)
MsgBox("Done")
Catch ex As Exception
MsgBox(ex.ToString())
End Try
Me.Close()
End Sub
End Class

What I have tried:

With very minor modification, this works on Outlook and Gmail servers (just Earthlink has gone hay-wire). I've tried changing passwords, order of server properties access via alternative e-mail accounts and more. Any ideas?