Exchange 2013 add email address with domain that isn't in accepted domains yet

Gentlemen.

We have purchased a company, and they will eventually be using our exchange server. We don't have control of their domain yet, but when we do we will change the MX records to point to our exchange server and add it to the list of accepted domains.

I've added all their domain accounts and email boxes, and just gave them our domain as an email address temporarily with the idea of going into exchange and adding the domain to the list of accepted domains when the MX records propagate and mail starts flowing here and changing their email address to that domain.  My question is this: I don't want to add that domain yet to our accepted domain list because we still are sending email to a couple of people in that domain regularly, but for everybody else who won't even log into their box on our exchange server until we go live will it hurt anything if I just gave them their eventual email address now even if mail to that domain isn't flowing to our exchange server yet (I noticed that exchange server allows me to put anything I want for a new email address) and then simply add the domain to the list of accepted domains when we go live?

I guess the question in a nutshell is: does it hurt anything if I add an email address to a user in exchange if the address's domain doesn't exist in the list of accepted domains yet?
twinsteadAsked:
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KimputerConnect With a Mentor Commented:
In that case, it won't hurt at all
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KimputerCommented:
It doesn't hurt, in the sense that:

if you set it as primary, the other side who receives the email might already click reply, and that reply will go into the dark abyss.
If you set it as an extra email address, nothing happens (the other side won't notice you added an extra email.).
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twinsteadAuthor Commented:
I don't think that will be an issue because none of these users will ever connect to these boxes and use them to send mail until we are live and mail for the domain is being directed to our exchange server and the domain is added to the accepted domain list.
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twinsteadAuthor Commented:
Perfect. Thank you for your help sir
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Todd NelsonSystems EngineerCommented:
Going through this right now with a merge of 3 companies.  Basically we did it like this...

  • All mailboxes must send as a parent company email address.
  • On acquired companies' Exchange servers, disabled application of EAP and added new primary address of parent company email address.
  • On parent Exchange servers, create contacts (hidden) set with acquired companies' old addresses.
  • On parent Exchange servers, create mailboxes for acquired companies' users and set forwarding to associated contact.
  • Finally, updated the SPF record for the parent company with the acquired company outbound IP addresses.

As a result, mail goes out of the acquired company with the parent company address, then comes into the parent company, and is immediately forwarded to the acquired company mailboxes.

We did not add any accepted domains.

All in preparation for a email migration later this year.

Hope this helps.
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