Exchange E-mail Address Policy – Force lowercase email address and change current

Since the policy grabs data from AD in whatever case it is in, you need to apply this custom prefix, which basically says Replace A for a, B for b etc

%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

You can use this with firstname.Surname@yourdomain.com thusly:

%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@yourdomain.com

Sweet.  But it will not change the case of current email addresses.  Dunno why.

For this you can use this powershell PS1 script

$MailboxList = Get-Mailbox -ResultSize unlimited

$MailboxList | % {

$LoweredList = @()
$RenamedList = @()

foreach ($Address in $_.EmailAddresses){
if ($Address.prefixstring -eq "SMTP"){
$RenamedList += $Address.smtpaddress + "TempRename"
$LoweredList += $Address.smtpaddress.ToLower()
}
}
Set-mailbox $_ -emailaddresses $RenamedList -EmailAddressPolicyEnabled $false
Set-mailbox $_ -emailaddresses $LoweredList

#Without this line the "Reply To" Address could be lost on recipients with more than one proxy address:
Set-mailbox $_ -PrimarySmtpAddress $_.PrimarySmtpAddress
}