SMTP configuration SendUsing configuration value is invalid with ASP-Classic

If you are specifying an smptserver, be sure to set your 'sendusing' field to 2 (or cdoSendUsingPort) as well:

objMsg.Configuration.Fields.Item("http://schemas.microsoft.com/cdo/configuration/sendusing") = 2
objMsg.Configuration.Fields.Item("http://schemas.microsoft.com/cdo/configuration/smtpserver") = "localhost"
objMsg.Configuration.Fields.Item("http://schemas.microsoft.com/cdo/configuration/smtpserverport") = 25
objMsg.Configuration.Fields.Update 

As the SMTP service is on the localhost it makes more sense to send to pickup directory using SendUsingPickup (1). This will be more efficient than sending over network to port 25.

objMsg.Configuration.Fields.Item("http://schemas.microsoft.com/cdo/configuration/sendusing") = 1
objMsg.Configuration.Fields.Item("http://schemas.microsoft.com/cdo/configuration/smtpserverpickupdirectory") = "C:\Inetpub\mailroot\Pickup"
objMsg.Configuration.Fields.Update