You can log replies from your leads right in Outlook with the Marketo Email Add-In.
From the Main Outlook Application
Select the email you want to log and click Log with Marketo.
You can also right-click on a message and click Log With Marketo.
You should see a confirmation.
From the Email Itself
If you have opened the email, you can simply click on the Log with Marketo button from there.
You should see the same confirmation as the other method.
Log your lead's replies to add them to their history in Marketo.