You can't normally do this due to the nature of how email is processed. When you send an email, it is sent to the receiver's pop server (so they can retrieve it). But YOU have no control over email once it's sent, neither while in transit nor eventually in the hands of the recipient’s pop server. It's sort of like what happens when you drop a regular letter in the mailbox -- at that point, it becomes the physical property of the USPS. There simply is no smtp/pop protocol that would permit you to delete *ANY* mail while in transit or once it resides w/the recipient's pop server (i.e., their mailbox). That's just the way it works.