Te faltan las directivas de configuración antes de mail.send
Código:
objMessage.Configuration.Fields.Item("http://schemas.microsoft.com/cdo/configuration/sendusing") = 2
objMessage.Configuration.Fields.Item("http://schemas.microsoft.com/cdo/configuration/smtpserver") = "smtp.servidor.com"
objMessage.Configuration.Fields.Item("http://schemas.microsoft.com/cdo/configuration/smtpserverport") = 25
objMessage.Configuration.Fields.Item("http://schemas.microsoft.com/cdo/configuration/smtpconnectiontimeout") = 60