users not receiving mails from external

i have a domain called college as a public registered domain and i am having an exchange in my infrastructure. and created an mx record poining to my exchange server

i need to create a new domain name for my students to use the mail address. So i have decided to create a subdomain in my external dns called, so that my student mail id will like and need to point an mx record to microsoft windows live server , exchange labs

now when a user send a mail from outside to my student mails, it gives an NDR , unknown host

i think this is an error with DNS, could anyone help me to setup up the subdomain in my external dns

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mishalkConnect With a Mentor Author Commented:
it was the error related to the host record in external dns
what happes if you telnet to the mail server on port 25, and try to send an email to that address?
mishalkAuthor Commented:
dear hodgeyohn: the mail is hosted in microsoft , we are using windows live@edu facility.
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