C program verifies whether the email address actually exists (not the authentication email format)

Principle: connect to the domain name email server and simulate sending an email to the recipient. After verifying the result to rcpt to, the next address will continue without actually sending the email. Compiled and tested in REDHAT

The actual results will be saved in the file "OK", and those that do not exist will be saved in "bad", because the recipient's email server may also cause some mailbox detection results to be incorrect sometimes, therefore, inaccurate information is stored in the unkown file, which can be verified again for more accurate filtering.

The source code is included in the attachment at the end of the article.

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