hai impostato due volte il campo sendusing, cancella questa riga:

.Fields("http://schemas.microsoft.com/cdo/configuration/sendusing") = 1


e prova a sostituire queste righe:

.Fields("http://schemas.microsoft.com/cdo/configuration/smtpauthenticate")=1

.Fields("http://schemas.microsoft.com/cdo/configuration/sendusername") = "info@miosito.org"

.Fields("http://schemas.microsoft.com/cdo/configuration/sendpassword") = "xxxxxxxx"