That transmitted document is no less secure than the contents of a normal telephone conversation. That computer is almost certainly owned and operated by a third party. They run software that helps your e-mail message reach its intended destination. There are two threats to the confidentiality of e-mail messages on a router. First, the system administrator of the router may read the message.
System administrators can access e-mail being sent over their computers.
Ordinarily, computers ignore data not intended for them. Sniffer software tells a computer to accept all data sent in that circuit, not just data addressed to that computer. The sniffer can then filter out data to search for packets addressed to a specific machine. Spoofer software allows a computer to act like the recipient's computer. When packets arrive, the spoofer sends back a message saying that the information has reached its destination.