Ok, ho risolto sistemandolo così

codice:
<%
Dim form, subject, allegato
%>

<Html>
<head>
<title>Email spedita</title>
</head>
<body>



<%
Dim oUpload 
Set oUpload = new cUpload
oUpload.SetPath "upload\"
oUpload.Load
if oUpload.Count=0 then
	allegato = ""
Else
	While Not oUpload.EOF
		oUpload.Save()
                allegato = oUpload.GetFileName()
		oUpload.MoveNext
	Wend
End if



call Main()



Sub Main()
If Len(oUpload.Form("subject")) = 0 then
no_sub()
else 
manda_posta(form)
ecco_fatto(nome)
end if
Set oUpload=Nothing 
End sub



Sub no_sub()
response.write "<div align='center'>"
response.write "<table border='0' width='80%' cellspadding='4' cellspacing='4'>" 
response.write "<tr><td>Non hai inserito un subject</td></tr>"
response.write "<tr><td>Per favore, torna indiedro e inseriscine uno </td></tr>"
response.write "<tr><td align='middlè>"
response.write "<a href=Javascript:history.go(-1)>Pagina precedente</a>"
response.write "</td></tr></table></div>"
End Sub



Sub manda_posta(form)
Dim subject, testo
Dim objMail, objMailConfig
subject = oUpload.Form("subject")
mittente = oUpload.Form("mittente")
testo = oUpload.Form("testo")
Set objMail = Server.CreateObject("CDO.Message")
Set objMailConfig = Server.CreateObject ("CDO.Configuration")

' Proprietà di invio
with objMailConfig
  ' SMTP mail server
  .Fields("http://schemas.microsoft.com/cdo/configuration/smtpserver") = "smtp.fastwebnet.it"
  ' SMTP port
  .Fields("http://schemas.microsoft.com/cdo/configuration/smtpserverport") = 25
  ' CDO port
  .Fields("http://schemas.microsoft.com/cdo/configuration/sendusing") = 2
  ' Timeout
  .Fields("http://schemas.microsoft.com/cdo/configuration/smtpconnectiontimeout") = 60
  .Fields.Update
end with

Set objMail.Configuration = objMailConfig

objMail.From = "mia_mail@hotmai.it"
objMail.To = oUpload.Form("destinatario")
objMail.Subject = subject
objMail.TextBody = testo
If Len(allegato) <> 0 then
objMail.AddAttachment Server.MapPath("upload\" & allegato)
end if
objMail.Fields("urn:schemas:httpmail:importance").Value = 0
objMail.Fields.update() 
objMail.Send
Set objMail = Nothing
end sub



Sub ecco_fatto(nome)
response.write "<div align='center'>"
response.write "<table border='0' width='80%' cellspadding='4' cellspacing='4'>" 
response.write "<tr><td>Grazie " & subject & " per la mail</td></tr>"
response.write "<tr><td align='middlè>"
response.write "<a href=Javascript:history.go(-2)>Torna indietro</a>"
response.write "</td></tr></table></div>"
End sub



%>
</body>
</html>
Grazie, Roby.

Solo due domande:
1) Così come ho fatto va bene? C'è qualcosa che ho fatto a "modo mio" ma che si dovrebbe/potrebbe fare meglio?
2) Una volta spedito il file, mi servirebbe che venisse eliminato dal server: potete dirmi come?



Ringrazio in anticipo ^^