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

If you already have a list of accounts, importing a CSV can make the operation more controlled:

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

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

if ($adUser.proxyAddresses -notcontains $alias) {
Set-ADUser $adUser -Add @{proxyAddresses=$alias}
}
}

Before running this broadly, export the current proxyAddresses values, test with a small group, and check for duplicate aliases, missing names, and accounts that already have the desired address. If the alias needs to become primary, that is a separate change because the existing uppercase `SMTP:` entry must be replaced carefully.

For a basic single-user example, you can read the first name and surname, build the alias, and add it to proxyAddresses:

Get-ADUser jdoe -Properties GivenName,Surname | ForEach-Object {
$alias = "smtp:$($_.GivenName).$($_.Surname)@domain2.co.uk"
Set-ADUser $_ -Add @{proxyAddresses=$alias}
}

Use lowercase `smtp:` for a secondary alias. The uppercase `SMTP:` prefix is normally reserved for the primary address, so don’t use it unless you intend to change the primary email address.

Avoid putting real employee data or directory exports into an external AI service. A safer approach is to work from a controlled CSV or directly query the directory, then use PowerShell string manipulation to construct the alias. You can also split an existing email address at the `@` symbol if that better matches your naming rules, but validate the generated values before writing them back to Active Directory.

Break the task into smaller steps: retrieve the users with Get-ADUser or the appropriate mailbox cmdlet, filter the accounts you actually want to change, loop through them, update the proxyAddresses attribute, and then validate the results. It’s best to get the command working against one test account first before scaling it to the whole directory.