Hi Chloe,
I'm assuming you have two tables Client (parent) and Emails (child). If that's the case, you can create a summary field on the relationship that looks for the Max of the Date/Time of the Email. You can also filter by "sent" or "received" if you have such a field. Creating one summary each for either "sent" or "received".
If you don't have such a field, you can create a formula that looks for the 'from' email address for your company and marks it as "sent", otherwise "received".
Let me know if this makes sesnse, I can provide more details if necessary.
-Sharon