Enabling Relay Permissions on SMTP Virtual Server Properties

We have a user who is having a problem sending e-mails externally. When sending this e-mail the user gets a bounce back with the following message;

 Subject:      RE: Ref 001 / HR1
      Sent:      07/11/2007 18:11

The following recipient(s) could not be reached:

      Wallace, Tanya on 07/11/2007 18:11
            You do not have permission to send to this recipient.  For assistance, contact your system administrator.
            MSEXCH:MSExchangeIS:/DC=uk/DC=co/DC=hertel:HTMAIL

I have looked into this and have been advised to enable Relay Permission within the SMTP Virtual Server properties. See below;

ESM->Administrative Groups->First Administrative Group->Servers->[MyServer]->Protocols->SMTP->Default SMTP Virtual Server->Proporties->Access->Authentication->Users and grant Authenticated Users the Relay Permission.

This issue only seems to be effecting one user, and by enabling these permissions will apply to all our exchange users. Can somone advise why this could be the fix and also advise what potential risk if any I would be taking be enabling this serviceStart Free Trial