Fixed Symfony TransportException: Email failed: Expected response code "250" but got code "451", with message "451", with message "451 4.4.2 Timeout

[Alpha1]

Affected version

2.3.10

I have loads of the following errors. I had these on the previous server and now on ultrafast server these keep coming. AWS SES sending rate limits have been royally increased, so its not that.

Symfony\Component\Mailer\Exception\TransportException: Email to xxx@gmail.com from xxx@forum.com failed: Expected response code "250" but got code "451", with message "451 4.4.2 Timeout waiting for data from client.".

• src/vendor/symfony/mailer/Transport/Smtp/SmtpTransport.php:307
• Generated by: Unknown account
  May 12, 2026 at 1:00 PM

#0 src/vendor/symfony/mailer/Transport/Smtp/SmtpTransport.php(182): Symfony\Component\Mailer\Transport\Smtp\SmtpTransport->assertResponseCode('451 4.4.2 Timeo...', Array)
#1 src/vendor/symfony/mailer/Transport/Smtp/SmtpTransport.php(235): Symfony\Component\Mailer\Transport\Smtp\SmtpTransport->executeCommand('MAIL FROM:<xxx...', Array)
#2 src/vendor/symfony/mailer/Transport/Smtp/SmtpTransport.php(199): Symfony\Component\Mailer\Transport\Smtp\SmtpTransport->doMailFromCommand('xxx@...')
#3 src/vendor/symfony/mailer/Transport/AbstractTransport.php(72): Symfony\Component\Mailer\Transport\Smtp\SmtpTransport->doSend(Object(Symfony\Component\Mailer\SentMessage))
#4 src/vendor/symfony/mailer/Transport/Smtp/SmtpTransport.php(136): Symfony\Component\Mailer\Transport\AbstractTransport->send(Object(Symfony\Component\Mailer\SentMessage), Object(Symfony\Component\Mailer\DelayedEnvelope))
#5 src/XF/Mail/Mailer.php(470): Symfony\Component\Mailer\Transport\Smtp\SmtpTransport->send(Object(Symfony\Component\Mime\Email))
#6 src/addons/DBTech/Mail/XF/Mail/Mailer.php(60): XF\Mail\Mailer->send(Object(Symfony\Component\Mime\Email), Object(Symfony\Component\Mailer\Transport\Smtp\EsmtpTransport))
#7 src/addons/DigitalPoint/Analytics/XF/Mail/Mailer.php(113): DBTech\Mail\XF\Mail\Mailer->send(Object(Symfony\Component\Mime\Email), NULL)
#8 src/XF/Job/MailSend.php(37): DigitalPoint\Analytics\XF\Mail\Mailer->send(Object(Symfony\Component\Mime\Email))
#9 src/addons/DBTech/Mail/XF/Job/MailSend.php(26): XF\Job\MailSend->run(44.7112)
#10 src/XF/Job/Manager.php(275): DBTech\Mail\XF\Job\MailSend->run(44.7112)
#11 src/XF/Job/Manager.php(205): XF\Job\Manager->runJobInternal(Array, 44.7112)
#12 src/XF/Job/Manager.php(89): XF\Job\Manager->runJobEntry(Array, 44.7112)
#13 src/XF/Cli/Command/RunJobs.php(58): XF\Job\Manager->runQueue(false, 45)
#14 src/vendor/symfony/console/Command/Command.php(298): XF\Cli\Command\RunJobs->execute(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#15 src/vendor/symfony/console/Application.php(1040): Symfony\Component\Console\Command\Command->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#16 src/vendor/symfony/console/Application.php(301): Symfony\Component\Console\Application->doRunCommand(Object(XF\Cli\Command\RunJobs), Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#17 src/vendor/symfony/console/Application.php(171): Symfony\Component\Console\Application->doRun(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#18 src/XF/Cli/Runner.php(115): Symfony\Component\Console\Application->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#19 cmd.php(17): XF\Cli\Runner->run()
#20 {main}

array(1) {
["cli"] => string(62) "/cmd.php xf:run-jobs"
}

Similar bug report here which seems to have lead to the switch to Symphony:

Fixed Thread 'ErrorException during email sending'

Mar 30, 2021

Hey, we've been monitoring 3 individual errors appear frequently in the ACP Server Error Log:

ErrorException: Email to user@example.com failed: [E_WARNING] fwrite(): SSL: Broken pipe src/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/StreamBuffer.php:240

Swift_TransportException: Email to user@example.com failed: Expected response code 250 but got code "451", with message "451 4.4.2 Timeout waiting for data from client. " src/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php:383

Email to user@example.com failed: Expected response code...

• Externalizable
• Replies: 24
• Forum: Resolved bug reports

• 

Some related information:

https://github.com/symfony/symfony/issues/45586

This seems to suggest that xenforo connection through Symfony mail goes idle, leading to this error.

 

[Jake B.]

We've started having this come up in pretty much a constant stream after upgrading a client's site to XF 2.3.10 from 2.2.x, also using Amazon SES

 

[Jake B.]

Have applied this patch that seems to address the issue, so far:

diff --git a/src/XF/Mail/Mailer.php b/src/XF/Mail/Mailer.php
index a058c9dbb2..b13b4bb47a 100644
--- a/src/XF/Mail/Mailer.php
+++ b/src/XF/Mail/Mailer.php
@@ -561,6 +561,11 @@ class Mailer
                     $stream->disableTls();
                 }
 
+                $pingThreshold = (int) (\XF::app()->config('smtpKeepAlivePingThreshold') ?: 5);
+                if ($pingThreshold > 0) {
+                    $transport->setPingThreshold($pingThreshold);
+                }
+
                 return $transport;
 
             case 'file':

Defaults to 5 seconds, but can be adjusted with:

$config['smtpKeepAlivePingThreshold'] = 30;

have attached an add-on as well that does the same thing without manual code changes in case this never gets patched for some reason.

Edit: 4 hours later, nothing logged still, prior to this patch it was being logged every minute or so

 

[XF Bug Bot]

Thank you for reporting this issue, it has now been resolved. We are aiming to include any changes that have been made in a future XF release (2.3.11).

Change log:

Ping idle SMTP keep-alive connections before sending to avoid 451 timeout errors

There may be a delay before changes are rolled out to the XenForo Community.

 

[Chris D]

Hi @Jake B. after a review we adopted your suggested fix with the only adjustment being that the default timeout (same config key) actually defaults to 9 seconds as this seems to be a safe and recommended default. That being said, adjustable as per the config example above.

Thanks @Jake B.!

 

[Alpha1]

Many thanks @Jake B. and @Chris D This one has been doing my head in

 

[nocte]

Thanks a lot @Jake B.!

The add-on (prior to upgrading to XF 2.3.11) fixed a similar issue for me:

XF 2.3 Thread 'Every 10 minutes server error on mail send via cron: "Unable to write bytes on the wire"'

Mar 12, 2026

Since the upgrade from 2.3.7 to 2.3.10 we get this error roughly every 10 minutes (i.e. minute 0, 10, 20, 30, 40, 50) once:

Symfony\Component\Mailer\Exception\TransportException: Email to ****gmail.com from ***@forum.tld failed: Unable to write bytes on the wire. src/vendor/symfony/mailer/Transport/Smtp/Stream/AbstractStream.php:47
Generated by: Unknown account 12.3.2026 at 16:50
Stack trace
#0 src/vendor/symfony/mailer/Transport/Smtp/SmtpTransport.php(180): Symfony\Component\Mailer\Transport\Smtp\Stream\AbstractStream->write('MAIL FROM:<boun...')
#1...

• nocte
• Replies: 0
• Forum: Troubleshooting and problems