Outlook and 5.5 exchange server problems.

We are running Microsoft Exchange 5.5.  A user has set up his mail in Outlook in Bellsouth.net on his PC.  He tries to send an email to our office mail server and it is immediately returned to him stating SERVER DOES NOT ACCEPT MAIL when using Outlook.  The same user goes directly to the WEB MAIL SERVER for Bellsouth.net and the mail comes into the mail server immediatley with no return message.  This is happening with everyone that is using Outlook to send mail to our office.  We need help....
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Pushpakumara MahagamageConnect With a Mentor VPCommented:
if he use pop or imap he should login your smtp server

so tools email accounts properties of your company account   more settings or advanced option    server tab

check my smto server requared athontication

if you can paste NDR(return message)  here

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