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503 Bad sequence of commands if count($this->all_recipients) == count($bad_rcpt) · Issue #156 · PHPMailer/PHPMailer · GitHub
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![SMTP Basics: How It Works and Why It Matters [2023]](https://mailtrap.io/wp-content/uploads/2023/03/SMTP-Model-from-RFC821.png "SMTP Basics: How It Works and Why It Matters [2023]")
SMTP Basics: How It Works and Why It Matters [2023]
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