[#ZBX-23694] plain text emails timeout (resulting in 3 sent mails)

For normal problem triggers, they are sent 3 times. I'm guessing because of a retry-mechanism built into the server. And then they still show as failed.
Even though I got 3 mails.

tcpdump from the server where I did the test via the interface (tcp.stream eq 0):

When I run a simple php script mimicking the zabbix flow observed, I get the result in "tcp.stream eq 3":

<?php
// SMTP server settings
$smtpServer = "mail.salvania.be";
$smtpPort = 25;

// Sender and recipient details
$from = "[email protected]";
$to = "[email protected]";

// Email content
$message = "From: \"Zabbix\" <[email protected]>\r\n".
"To: <[email protected]>\r\n".
"In-Reply-To: <0.e6aaeb1c51ec501da39a0f92f4e6e84e.1.72717f1bd11d285a2415c519590d68d2@zabbix.com>\r\n".
"Date: Fri, 10 Nov 2023 09:23:23 +0100\r\n".
"Subject: Test subject\r\n".
"MIME-Version: 1.0\r\n".
"Content-Type: text/plain; charset=\"UTF-8\"\r\n".
"Content-Transfer-Encoding: base64\r\n".
"\r\n".
'VGhpcyBpcyB0aGUgdGVzdCBtZXNzYWdlIGZyb20gWmFiYml4';

// Function to send command and receive response
function sendCommand($smtpConnection, $command, $fetch = true) {
    foreach( explode("\n", rtrim($command)) as $line ) {
        $line = rtrim($line);
        echo "> {$line}\n";
    }

    fputs($smtpConnection, $command);
    if ($fetch)
        echo "< " . fgets($smtpConnection);
}

// Open a TCP connection to the SMTP server
$smtpConnection = fsockopen($smtpServer, $smtpPort, $errno, $errstr, 10);

// Check if the connection is successful
if (!$smtpConnection) {
    die("Failed to connect to the SMTP server: $errstr ($errno)");
}

// Read the welcome message from the SMTP server
echo "< " . fgets($smtpConnection);

// Send the EHLO command and handle multi-line response
sendCommand($smtpConnection, "HELO salvania.be\r\n");

// Send the MAIL FROM command
sendCommand($smtpConnection, "MAIL FROM: <$from>\r\n");

// Send the RCPT TO command
sendCommand($smtpConnection, "RCPT TO: <$to>\r\n");

// Send the DATA command
sendCommand($smtpConnection, "DATA\r\n");

// Send the email headers and body
sendCommand($smtpConnection, $message, false);
sendCommand($smtpConnection, "\r\n.\r\n");

// Send the QUIT command
sendCommand($smtpConnection, "QUIT\r\n");

// Close the connection
fclose($smtpConnection);
?>

[ZBX-23694] plain text emails timeout (resulting in 3 sent mails) Created: 2023 Nov 10 Updated: 2023 Nov 15 Resolved: 2023 Nov 14
Status:	Closed
Project:	ZABBIX BUGS AND ISSUES
Component/s:	Server (S)
Affects Version/s:	6.4.8
Fix Version/s:	None

[ZBX-23694] plain text emails timeout (resulting in 3 sent mails) Created: 2023 Nov 10 Updated: 2023 Nov 15 Resolved: 2023 Nov 14