Suppose you work at an organization that is about to change it’s primary e-mail domain and that organization doesn’t use e-mail address policies. Changing e-mail domains isn’t unlikely because of mergers or rebranding and yes, there are organizations out there not utilizing e-mail address policies for various reasons.

This post will describe what steps are involved and how to accomplish this task using PowerShell. I’ll use a phased approach since organizations performing all these steps at once are rare. Note that examples below will use oldcorp.com for the old e-mail domain and newcorp.com for the new e-mail domain. 

First, we’ll start off by adding the new domain to the list of accepted e-mail domains in the Exchange organization. We’re authoritative for the new e-mail domain, so use the the New-AcceptedDomain cmdlet in conjunction with the Authoritative as DomainType to add this e-mail domain using Exchange Management Shell:

New-AcceptedDomain -Name "New Corp" -DomainName newcorp.com -DomainType Authoritative

Next, configure the MX records for the newcorp.com domain on DNS. Let them point to your inbound SMTP server or appliance and, if required, configure your anti-spam/anti-virus solution accordingly.

Now for the more interesting part: how to add the new e-mail domain to objects and make the switch from oldcorp.com to newcorp.com in bulk. We’ll start by adding newcorp.com proxy addresses (the EmailAddresses property) for all existing olddomain.com e-mail addresses using Set-Mailbox, using the current user part (the part before the “@”). Take caution when adding an e-mail address to EmailAddresses, since EmailAddresses is an array and you could end up overwriting all current proxy addresses with a single element when used like this:

Set-Mailbox mderooij -EmailAddresses michel.de.rooij@newcorp.com

Instead, we need to utilize the Add method. All roads lead to Rome, so I’ll pick the most efficient manner as shown in the script below. Be advised that the script as shown processes all mail-enabled objects which primary address is @oldcorp.com (as a form of filtering). Therefore, take caution and use it in your test environment first when appropriate.

Get-Mailbox -ResultSize Unlimited | Where { $_.WindowsEmailAddress -like "*@oldcorp.com" } | ForEach { 
    $This= $_
    $_.EmailAddresses | where { $_.SmtpAddress -like "*@oldcorp.com" } | ForEach {
        $NewMailAddress= $_.SmtpAddress.ToString().Split( "@")[0] + "@newcorp.com"
        "Adding $NewMailAddress for "+ $This.WindowsEmailAddress
        Set-Mailbox $_.Identity -EmailAddresses @{Add=$NewMailAddress}
    }
}

Now, inbound e-mail for newcorp.com is accepted and accounts are configured to receive e-mail on the new newcorp.com e-mail domain.

Add a certain point in time, we’ll make the switch from oldcorp.com to newcorp.com, so all outbound e-mail will use the new e-mail address. To do this, we need to use the Set-Mailbox cmdlet in conjuction with the PrimarySmtpAddress parameter. We’ll only look at the WindowsEmailAddress, which holds the current Primary SMTP address:

Get-Mailbox -ResultSize Unlimited | Where { $_.WindowsEmailAddress -like "*@oldcorp" } | ForEach {
    $ID= $_
    $NewPrimary= $_.WindowsEmailAddress.ToString().Split( "@")[0] + "@newcorp.com"
    "Promoting "+ $NewPrimary
    Set-Mailbox $ID -PrimarySmtpAddress $NewPrimary
}

The last thing to do after determining everything is working fine and everyone is used to the new e-mail domain is removing those @olddomain.com secondaries. We’ll take the current set of EmailAddresses, filter on not having the olddomain.com e-mail domain, and put the filtered result back in the EmailAddress property.

Get-Mailbox -ResultSize Unlimited | Where { $_.WindowsEmailAddress -like "*@newcorp.com" } | ForEach {
    $NewSecondaries= ($_.EmailAddresses | where { $_.SmtpAddress -notlike "*@oldcorp.com" })
    Set-Mailbox $_ -EmailAddresses $NewSecondaries
}

I hope you find this information useful. If you have questions, do not hesitate asking me in the comments.