wp-includes/PHPMailer/SMTP.php:1584Extract and return the ID of the last SMTP transaction based on a list of patterns provided in SMTP::$smtp_transaction_id_patterns.
bool|string|null protected function recordLastTransactionID() { $reply = $this->getLastReply(); if (empty($reply)) { $this->last_smtp_transaction_id = null; } else { $this->last_smtp_transaction_id = false; foreach ($this->smtp_transaction_id_patterns as $smtp_transaction_id_pattern) { $matches = []; if (preg_match($smtp_transaction_id_pattern, $reply, $matches)) { $this->last_smtp_transaction_id = trim($matches[1]); break; } } } return $this->last_smtp_transaction_id; }Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/PHPMailer/SMTP.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.