Sender Callout Exchange

• A mail host is a computer on the internet that is dedicated to transferring e-mail from senders to recipients.
• Headers identify the sender and recipient of a message, the route the message took from one site to another and so on.
• A challenge-response filter is a spam filter that works by blocking unknown senders mail from delivery until they prove that they are well-intentioned human beings and not spammers or robots.
• Spam continues to grow as senders find motivation to increase their efforts.
• a method of blocking spam which requires the sender of the email to manually respond to a verification message sent to them.
• Sender verification callout: A spam-prevention system in which the identity of the sender of incoming email is verified before the email is accepted.
• The standard method is for the receiving system to connect to the mail server specified in the "from:" header and verify the sender's identity.
• Then a "challenge" message is automatically sent back to the sender, asking for a response, and if such a response is received, the sender's original message is delived.
• Challenge Response: A spam-prevention system in which the first email contact from an unknown sender is bounced back with a "prove this is really you" message.
• Once the sender jumps through whatever hoop the challenge-response system requires, they are allowed to send mail to the recipient.
• Better c-r systems will queue the original message, and send it through once the sender has validated themself, saving the sender the trouble of retransmitting the message.
• Although commonly used as an anti-spam system, and available with a number of mail transfer agents, sender verification callout has many of the same problems as challenge-response.
• Since most spam has forged "from:" lines, the verification step will result in an unwanted connection to the sender's system, tying up bandwidth of an innocent third party.

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