How can I bulk-add new-format email aliases to Active Directory users with PowerShell?

If you already have a list of accounts, import it from CSV and process each one. Include checks so you don’t add duplicates or overwrite existing values:

$legacyUsers = Import-Csv 'C:legacyUsers.csv'

foreach ($account in $legacyUsers) {
$user = Get-ADUser -Identity $account.SamAccountName -Properties GivenName,Surname,proxyAddresses
$alias = "smtp:$($user.GivenName).$($user.Surname)@domain2.co.uk"

if ($user.proxyAddresses -notcontains $alias) {
Set-ADUser -Identity $user -Add @{proxyAddresses=$alias}
Write-Host "Added $alias to $($user.SamAccountName)"
}
}

Test it on a small group first, and consider using `-WhatIf` where supported or replacing the update with output to a CSV so you can review the proposed changes.

For a single account, you can construct the alias from the first name and surname and add it as a lowercase secondary address. For example:

$adUser = Get-ADUser -Identity 'jdoe' -Properties GivenName,Surname,proxyAddresses
$alias = "smtp:$($adUser.GivenName).$($adUser.Surname)@domain2.co.uk"
Set-ADUser -Identity $adUser -Add @{proxyAddresses=$alias}

The lowercase `smtp:` prefix marks it as a secondary alias. An uppercase `SMTP:` prefix is used for the primary address, so don’t use that unless you intentionally want to change the primary address.

Be careful about relying on an AI tool with real directory data or personally identifiable information. The script should be designed around your environment, and you should test against representative but non-sensitive accounts. Also account for edge cases such as missing surnames, hyphens, apostrophes, duplicate aliases, and users who already have a different address. A preview report and post-update validation are important before modifying hundreds of accounts.

Break the task into smaller steps: retrieve the users with Get-ADUser, select or filter the accounts you want to change, loop through them with foreach, update proxyAddresses with Set-ADUser, and then verify the results. It’s best to make the command work for one test account first, run it in report or preview mode, and only then scale it up.