On Windows Mail it works fine.
Moreover, that problem appeared even when I tested the code on my local
machine through visual studio.

Anyhow, incredibly, I edited the code as follows:

Public Shared Function QuickSendEmail(ByVal subject As String, ByVal body As
String, ByVal isHTML As Boolean, ByVal recipient As String) As Boolean
       Dim msg As System.Net.Mail.MailMessage = New
System.Net.Mail.MailMessage()
       msg.To.Add(recipient)
       msg.From = New MailAddress(myemail@mydomain.com, "Staff di The Total
Site .it", System.Text.Encoding.UTF8)
       msg.Subject = subject
       msg.SubjectEncoding = System.Text.Encoding.UTF8
       msg.Body = body
       msg.BodyEncoding = System.Text.Encoding.UTF8
       msg.IsBodyHtml = isHTML
       msg.Priority = MailPriority.Normal

       'Add the Creddentials
       Dim client As SmtpClient = New SmtpClient()
       client.Credentials = New System.Net.NetworkCredential(username,
password)
       client.Port = 587 'or use 465
       client.Host = "smtp.gmail.com"
       client.EnableSsl = True

       client.Send(msg)
       Return True
End Function

And now, it works perfectly :-|
Curious. Before I was using web.config to set up smtp parameters, meh :|

Bye,