Applications unable to send email to external address after migrating to Exchange 2007

Hello Experts,

Our organization has is in the process of migrating from Exchange 2000 to Exchange 2007.  All of our mailboxes have been migrated to Exchange 2007 and are working both internally and externally.  One problem we are having is applications created by our programmin staff are designed to send email to internal users and external users.  These applications send to the internal users just fine.  However, when the application trys to send to external users, we receive the following error:

System.Net.Mail.SmtpFailedRecipientException: {"Mailbox unavailable. The server response was: 5.7.1 Unable to relay"}
Data: {System.Collections.ListDictionaryInternal}
HelpLink: Nothing
InnerException: Nothing
Message: "Mailbox unavailable. The server response was: 5.7.1 Unable to relay"
Source: "System"
StackTrace: "   at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception)
   at System.Net.Mail.SmtpClient.Send(MailMessage message)
   at UnboundWebPage.SendEmail(String strEmailAddress, String strSubject, String strBody, Boolean blnSuppressMsg, String strFromEmailAddress, Literal& LiteralEmailMsg_Ctrl, String strServerName) in C:\projects\Local Net 2.0\UnboundWebPage\UnboundWebPage.vb:line 1030"
    TargetSite: {System.Reflection.RuntimeMethodInfo}

The applications use an AD account that is not disabled and does have a mailbox.

Any help with this would be greatly appreciated.
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ndalmolin_13Author Commented:
This appears to have done the trick.  Thanks for the quick response.
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