Field email counter Read

[joebordes]

When you are on a module you can only access fields that are on that module or on a module related directly. For example, on a contact, you can access all it's fields and all the fields of the account that it is related to. You cannot access fields with a "many" relation, like invoices or opportunities of that contact.
The same happens with emails, given one lead you can have many emails, so you can't use any field of the email in lead filters or workflows.
You have to work with the email module. Go to the emails module and you will see that you can filter on the access count field there, because that field belongs to that module.