Callback verification
technique used with SMTP to validate e-mail addresses
Callback verification, also known as callout verification or Sender Address Verification, is a technique used by SMTP software in order to validate e-mail addresses. The most common target of verification is the sender address from the message envelope (the address specified during the SMTP dialogue as "MAIL FROM"). It is mostly used as an anti-spam measure.
Purpose
Since a large percentage of e-mail spam is generated from forged sender ("mfrom") addresses, some spam can be detected by checking whether forging resulted in an invalid address, using this method.
A related technique is "call forwards", in which a secondary or firewall mail exchanger can verify recipients at the primary mail exchanger for the domain in order to decide whether the address is deliverable.
Process
The receiving mail server verifies the sender address, by verifying both parts of the sender address - the domain name (part after the @ character), and the local part (part before the @ character). The first step is to establish a successful SMTP connection to the mail exchanger for the sender address. The mail exchanger is found by looking up the MX records in the domain's DNS zone. The second step is to query the exchanger, and make sure that it accepts the address as a valid one. This is done in the same way as sending an email to the address, however the process is stopped after the mail exchanger accepts or rejects the recipient address.
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This entry incorporates text from “Callback verification” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.