Executing a task completion based on email sent

Executing a task completion based on email sent

We use email templates to send emails to prospects.  We forgot to mark a completed task "email sent" for those contacts we sent the email to, is there an efficient way we can lookup these contacts and do a mass "completed task" so there is a record in activities? We can run a report for the the contacts that received the email but we are lost as to how to mark a completed task for those contacts.