Converting Email Addresses to Lowercase in Exchange Online/Hybrid

How to convert existing primary email addresses to lowercase, as well as ensuring that any new mailboxes also have lowercase primary email addresses.

This can be useful because it looks tidier and some applications that use email for SSO may be case sensitive.

Step 1: Update Email Address Policy

In on-prem Exchange, update the default policy to use lowercase:

Set-EmailAddressPolicy -Identity "Default Policy" -EnabledPrimarySMTPAddressTemplate "%rAa%rBb%rCc%rDd%rEe%rFf%rGg%rHh%rIi%rJj%rKk%rLl%rMm%rNn%rOo%rPp%rQq%rRr%rSs%rTt%rUu%rVv%rWw%rXx%rYy%rZz%g.%s@domain.com"

Step 2: Convert Existing Users

Run this script from your on-prem Exchange server to convert existing remote mailboxes:

$ErrorActionPreference = "SilentlyContinue"

Connect-Exchange2016  # Your connection script here

$OU = "OU=Users,DC=domain,DC=com"
$ADusers = Get-ADUser -Filter 'enabled -eq $true' -SearchBase $OU -properties Mail | Sort-Object Mail

$TargetObjects = @()

foreach ($user in $ADusers) {
    $TargetObject = Get-RemoteMailbox -Identity $user.samaccountname | Where {$_.PrimarySmtpAddress.ToLower() -cne $_.PrimarySmtpAddress}
    if ($TargetObject) {
        $TargetObjects += $TargetObject
    }
}

if (!$TargetObjects) {
    Write-Host "No mailboxes found with uppercase characters, exiting" -ForegroundColor Yellow
} else {
    Write-Host "Total mailboxes with uppercase characters:" $TargetObjects.count
    
    foreach ($RemoteMailbox in $TargetObjects) {
        Write-Host "Old address: $($RemoteMailbox.PrimarySmtpAddress)"
        Write-Host "New address: $($RemoteMailbox.PrimarySmtpAddress.ToLower())"
        
        Set-RemoteMailbox $RemoteMailbox.Identity -PrimarySmtpAddress ("TMP-Rename-" + $RemoteMailbox.PrimarySmtpAddress) -EmailAddressPolicyEnabled $false
        Set-RemoteMailbox $RemoteMailbox.Identity -EmailAddresses @{remove = $RemoteMailbox.PrimarySmtpAddress}
        Set-RemoteMailbox $RemoteMailbox.Identity -PrimarySmtpAddress $RemoteMailbox.PrimarySmtpAddress.ToLower()
        Set-RemoteMailbox $RemoteMailbox.Identity -EmailAddresses @{remove = ("TMP-Rename-" + $RemoteMailbox.PrimarySmtpAddress)}
        Set-RemoteMailbox $RemoteMailbox.Identity -EmailAddressPolicyEnabled $true
    }
}

Note: The only impact we found in a live environment was that iOS Mail users were prompted for their password again. Outlook on desktop or mobile was unaffected.