Ciao.
Anche io ho lo stesso problema, ma l'errore è diverso. Mi dà:
501 Error: malformed authentication response .
Magari la causa potrebbe essere la stessa.....

Ecco il codice...

Try
Dim username As String = "aaa@pec.bbb.it"
Dim password As String = "ccc"
Dim tcpclient As New Sockets.TcpClient()

tcpclient.Connect("smtps.pec.aruba.it", 465)

Dim sslstream As New System.Net.Security.SslStream(tcpclient.GetStream())
sslstream.AuthenticateAsClient("smtps.pec.aruba.it")

Dim readBuffer(20048) As Byte

Dim sw As New System.IO.StreamWriter(sslstream)
Dim sr As New System.IO.StreamReader(sslstream)

sw.WriteLine("EHLO aaa@pec.bbb.it")
sw.Flush()

sw.WriteLine("AUTH LOGIN")
sw.Flush()

Dim controllo As Boolean = True

While (controllo)
Dim line As String = sr.ReadLine()

If (line.Equals("334 VXNlcm5hbWU6"))Then
sw.Write(username)
sw.Flush()
controllo = False
End If
End While

sw.WriteLine(password)
sw.Flush()

sslstream.Read(readBuffer, 0, 20048)

Dim str As String = System.Text.Encoding.ASCII.GetString(readBuffer)
TextBox2.Text = TextBox2.Text & str & vbCrLf

sw.WriteLine("Mail From:<aaa@pec.bbb.it>")
sw.Flush()
sw.WriteLine("RCPT TO:<xxx@yyy.it>")
sw.Flush()
sw.WriteLine("DATA ")
sw.Flush()
sw.WriteLine("This is test message from aruba")
sw.WriteLine(".")
sw.Flush()

sslstream.Read(readBuffer, 0, 20048)

str = System.Text.Encoding.ASCII.GetString(readBuffer)

Catch ex As Sockets.SocketException
MsgBox(ex.Message)
End Try

Sì, con System.Web.Mail funziona.....
E' strano che con Net.Mail non funzioni.....mi piacerebbe capire perchè...

Certo....
Sarei felice di capire il perchè.
A presto...spero... :)