MS Exchange X500

I have two forests both with exchange 2013 servers. We will call them EX1 and Ex2. I will be creating new AD accounts and associated mailboxes for users moving from EX1 to EX2. These accounts will obviously be empty when users first login. In order to ensure these users get their old email I will simply export their email from EX1 to PST and then import it into their associated mailboxes in EX2. I will also repoint the domain name pointing to EX1 to EX2 and create email alias to ensure they still receive any email sent to their old address. - Hope you're still following!

My question is after the migration if users hit the reply button to any of their old email will this cause an issue as I recall some problem previously with X500 addresses, thanks.
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you will face NDR issue as legacyexchangedn value will remain missing at target
to avoid this situation, you need to properly migrate those mailboxes by provisioning source users to target
3rd party tools are also available
you may use exchange native scripts and ADMT for same
The process is bot complex
i suggest you open GIGs for same
In short, you need to migrate source accounts to target and do not not create new accounts
Sid_FAuthor Commented:
New accounts must be created as they are two entirely different forests. Surely X500 addresses can be created in the new environment.
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Sid_FAuthor Commented:
Thinking about this a little more

1) John is a user in the old company. I am switching off the entire network in the old company and giving everyone new accounts and email in the new companies exchange.

2) John now has a completely new email in the new company. To allow for customers to still reach John I repoint his old domain to the new companies exchange and setup secondary SMTP address to match his old address.

3) I also export his email out of the old exchange server to PST and import it into his mailbox on the new exchange server.

Will I still have NDR issues? If so this would in theory mean I would need two exchangeLegacyDN's entries? Server is 2013
No you cannot create x500 addresses manually, you need to copy it from source

The scenario you mentioned last will work great

you will not face problem if user 1st time send email to john old email address as it don't have cached copy of his email address which ultimately points to old exchange server

You may face problem when user have cached copy of John's email ID, in that case if user types his old email address manually, then it will not use cached copy
The ideal solution to avoid such issues is to copy John's old ID legacy exchange DN value and copy it to his new account as X500 address
Sid_FAuthor Commented:
Is this only an issue for people who send email using an auto complete file? so if my users delete their auto complete file or setup a new outlook profile then the legacy DN's are not required?

I have tested this with two mailboxes and it would appear they are not affected because there is no historic NK2 file. Hopefully someone can confirm.
Yes, that is absolutely correct

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