Posting this as I always bloody forget it!

import-module ActiveDirectory
Get-ADUser -SearchBase 'ou=users,dc=company,dc=local' -Filter * | Foreach-Object{
$firstname = $_.givenname
$lastname = $_.surname
$domain = "@company.co.uk"
$emailaddress = $firstname + "." + $lastname + $domain
Set-ADUser -Identity $_ -Email $emailaddress
}