Your SMTP server may be requiring Authentication. you can send mail using aspmail through SMTP server requiring Authentication by setting the following properties.
objMail.Username = "username"
objMail.Password = "password"
Regarding - 503 Server encountered bad sequence of commands
Indicates (probable) that your ISP mail server did not recognized a command sent that is erroneous. Some temporary event prevents the successful sending of the message or an intermittent drop in network line connection that caused your mail client to send erroneous command and sending in the future may be successful.
For complete details please have a look into this URL :