Sender Callouts Exchange

• The from-address (for technical reasons) does not need to be that of the sender of the mail! In nearly all spam this address is absent, bogus, or stolen from an innocent party.
• A message bounce is when a mail delivery agent notifies the sender of an e-mail message using the return-path address of the message, that the message cannot be delivered because its for-address is not valid, or is incapable of receiving the mail.
• Spamarrest works by questioning unknown senders with its innovative challenge/response function.
• The unknown sender will receive the challenge request, where they need to respond.
• So, if an unknown sender fails to respond their email lands up in your junk email folder.
• Senders only have to be authenticated once by spamarrest, before being added to the safe list.
• One drawback is that some wanted mail may end up in the junk folder, if the sender doesn't respond to the spamarrest challenge mail.
• 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.
• This will cause all forged from: addresses to be accepted by the system trying to use sender verification callout.
• A for-address is considered the e-mail address that appears in the "For" clause in a typical e-mail header and it indicates the e-mail address to which the sender intends the message to be delivered.

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