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

[nocte]

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 src/vendor/symfony/mailer/Transport/Smtp/SmtpTransport.php(235): Symfony\Component\Mailer\Transport\Smtp\SmtpTransport->executeCommand('MAIL FROM:<boun...', Array)
#2 src/vendor/symfony/mailer/Transport/Smtp/SmtpTransport.php(199): Symfony\Component\Mailer\Transport\Smtp\SmtpTransport->doMailFromCommand('bounces+a154f9a...')
#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/XF/Job/MailSend.php(37): XF\Mail\Mailer->send(Object(Symfony\Component\Mime\Email))
#7 src/XF/Job/Manager.php(275): XF\Job\MailSend->run(18.4298)
#8 src/addons/Hampel/JobRunner/XF/Job/Manager.php(106): XF\Job\Manager->runJobInternal(Array, 18.4298)
#9 src/XF/Job/Manager.php(205): Hampel\JobRunner\XF\Job\Manager->runJobInternal(Array, 18.4298)
#10 src/addons/Hampel/JobRunner/XF/Job/Manager.php(96): XF\Job\Manager->runJobEntry(Array, 18.4298)
#11 src/XF/Job/Manager.php(89): Hampel\JobRunner\XF\Job\Manager->runJobEntry(Array, 18.4298)
#12 src/addons/Hampel/JobRunner/XF/Job/Manager.php(24): XF\Job\Manager->runQueue(false, 30)
#13 src/addons/Hampel/JobRunner/Cli/Command/RunJobs.php(98): Hampel\JobRunner\XF\Job\Manager->runQueue(false, 30)
#14 src/vendor/symfony/console/Command/Command.php(298): Hampel\JobRunner\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(Hampel\JobRunner\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}
Request state
array(1) {
  ["cli"] => string(71) "/home/forum/web/cmd.php --max-execution-time=238 hg:run-jobs -vvv"
}

We cannot identify any issues on the servers. We only upgraded XF, not the servers.

jobMaxRunTime is 30 seconds and it seems that the error occurs after 30 seconds run time of the cron task.