Electronic mail has a certain advantage because it is sent via a computer network; usually any error in sending the letter is caused by the person sending the mail. Email is never lost or stolen as it is always sent to the defined address. Email isn't all that private since it's sent through multiple servers, and server administrators can read and copy the message and its attachments if they want. Email does not guarantee that the message was sent by the person claiming to be sending the email. The postal service, on the other hand, has more reliability problems. The error is not uncommon as humans sort and deliver envelopes and packages. With this service, letters and packages are occasionally delivered to the wrong people. Thieves or grumpy postal workers can easily steal mail or packages from mailboxes if they feel like it. The Postal Service does not guarantee that a package will not be damaged or lost during shipping and is insured unless sent by registered mail at an additional cost. Authentication by post is more valid, since it is possible to see the signature and place anti-tampering devices on the envelope or package to highlight any tampering with the letter or package.