Thought this was simple, but not for me ;)

I have a customer.xls sheet and an orders.xls sheet. What I need to find
is the highest purchase date for each customer in customers.xls looking
up into orders.xls. Lookup is based on email address which is unique.

Even more simple is to find number of orders for each customer. Here I
thought that a countif would do the trick, but it just returns "0" :(