The following PowerShell script will check if any Active Directory users have an email address which contains an upper case character and will change all the email address to lowercase.

# Check email address is lowercase and update if not
Import-Module ActiveDirectory

$arrac = get-aduser -filter * -property SamAccountName, emailaddress

function CheckUpper {
   Param ($text)
   $c = 0
   $position = foreach ($character in $text.ToCharArray())
   {
     $c++
     if ([Char]::IsUpper($character))
     {
       return $c;
     }
   }
   return 0;
}

foreach($user in $arrac)
 {
    If ($user.EmailAddress -ne $null) {
        $sam = $user.SamAccountName
        $email = $user.EmailAddress
		if (CheckUpper($email) -gt 0) {
           $email2 = $user.EmailAddress.Tolower()
		   Write-host $sam "contains uppercase. converting $email -> $email2" -foreground red
           Set-ADUser -identity "$sam" -EmailAddress $email2
		}
		else {
		   Write-host $sam "only lowercase - $email" -foreground green
		}
    }
}

Needs to be run as a user with access to update the specified AD users.