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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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.
mishalkAuthor Commented:
it was the error related to the host record in external dns

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