change mailbox to external contact in exchange 2013


i need to change 10 mailbox to external contact.

i will delete this mailbox and create new external contact.

my question is:

i need to add x500 address or not?

because many user have already right a mail to this 10 mailbox.

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Yeah, I would add it to prevent feature NDR's.
Will SzymkowskiSenior Solution ArchitectCommented:
Yes providing the X500 address will allow user to reply to emails that they have previously received. This will also ensure that when the user is using AutoComplete in there Outlook client that they will not experience any NDR's.


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