如何確認收信者已 `收到信' 或 `已讀信'? - Google

knol中有一條是有關gmail能否有回條功能的說明

http://knol.google.com/k/read-receipts-in-gmail#

其中有解釋為何目前webmail多不提供回條的功能，內文貼上如下：


What's the problem with read receipts?

There are essentially two problems with read receipts in general:

They are unreliable: For a read receipt to work, the receiving
client must support it and be configured to use it. Many clients
(including most webmail services) will simply ignore any read
receipt request. For this reason, not receiving a read receipt
doesn't mean that the mail was not read. In addition, some systems
may reply to read receipts without the user having truly opened
the message. So receiving a read receipt doesn't necessarily mean
the mail was read. In other words, read receipts tell you almost
nothing. Even the sneakier techniques used by mail tracking services
are far less than perfectly reliable.

They may be considered a violation of privacy: Your recipient may
not want you to know exactly when they opened your email. For this
reason, most modern email clients won't send a read receipt without
the explicit confirmation of the recipient.

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