SMTP Mail Queueing improvements

Fixes #7787

Trying to keep inside the database schema, i'm abusing the priority level to slowly raise it to the max (127 due to signed tinyint).  It does this over a period of 3 days (which can be changed) by calculating the difference between the current time and the time the email was sent.

Sources/ScheduledTasks.php

@@ -712,7 +712,7 @@ function ReduceMailQueue($number = false, $override_limit = false, $force_send =
 
 	// Now we know how many we're sending, let's send them.
 	$request = $smcFunc['db_query']('', '
-		SELECT id_mail, recipient, body, subject, headers, send_html, time_sent, private
+		SELECT id_mail, recipient, body, subject, headers, send_html, time_sent, private, priority
 		FROM {db_prefix}mail_queue
 		ORDER BY priority ASC, id_mail ASC
 		LIMIT {int:limit}',
@@ -734,6 +734,7 @@ function ReduceMailQueue($number = false, $override_limit = false, $force_send =
 			'send_html' => $row['send_html'],
 			'time_sent' => $row['time_sent'],
 			'private' => $row['private'],
+			'priority' => $row['priority'],
 		);
 	}
 	$smcFunc['db_free_result']($request);
@@ -772,6 +773,8 @@ function ReduceMailQueue($number = false, $override_limit = false, $force_send =
 
 	// Send each email, yea!
 	$failed_emails = array();
+	$max_priority = 127;
+	$smtp_expire = ($modSettings['smtp_day_expiry'] ?? 3) * 60 * 60 * 24;
 	foreach ($emails as $email)
 	{
 		if (empty($modSettings['mail_type']) || $modSettings['smtp_host'] == '')
@@ -795,10 +798,16 @@ function ReduceMailQueue($number = false, $override_limit = false, $force_send =
 			$result = smtp_mail(array($email['to']), $email['subject'], $email['body'], $email['headers']);
 
 		// Hopefully it sent?
-		if (!$result)
-			$failed_emails[] = array($email['to'], $email['body'], $email['subject'], $email['headers'], $email['send_html'], $email['time_sent'], $email['private']);
+		if (!$result && $email['priority'] < $max_priority)
+		{
+			// Determine the "priority" as a way to keep track of SMTP failures.
+			$email['priority'] = max($email['priority'], min(ceil((time() - $email['time_sent']) / $smtp_expire * $max_priority), $max_priority));
+
+			$failed_emails[] = array($email['to'], $email['body'], $email['subject'], $email['headers'], $email['send_html'], $email['time_sent'], $email['private'], $email['priority']);
+		}
 	}
 
+
 	// Any emails that didn't send?
 	if (!empty($failed_emails))
 	{
@@ -826,7 +835,7 @@ function ReduceMailQueue($number = false, $override_limit = false, $force_send =
 		// Add our email back to the queue, manually.
 		$smcFunc['db_insert']('insert',
 			'{db_prefix}mail_queue',
-			array('recipient' => 'string', 'body' => 'string', 'subject' => 'string', 'headers' => 'string', 'send_html' => 'string', 'time_sent' => 'string', 'private' => 'int'),
+			array('recipient' => 'string', 'body' => 'string', 'subject' => 'string', 'headers' => 'string', 'send_html' => 'string', 'time_sent' => 'string', 'private' => 'int', 'priority' => 'int'),
 			$failed_emails,
 			array('id_mail')
 		);

Sources/Subs-Post.php

@@ -1592,9 +1592,10 @@ function server_parse($message, $socket, $code, &$response = null)
 	if ($code === null)
 		return substr($server_response, 0, 3);
 
-	if (substr($server_response, 0, 3) != $code)
+	if (($server_code = substr($server_response, 0, 3)) != $code)
 	{
-		log_error($txt['smtp_error'] . $server_response);
+		if ($server_code < 500 && !in_array($server_code, [450, 451, 421]))
+			log_error($txt['smtp_error'] . $server_response);
 		return false;
 	}