XF 2.0 Swiftmailer errors

[Mr Lucky]

Can someone tell me what this might be about: (thanks I got a few of these today)

Server error log

    Swift_TransportException: Email to xxx@xxxxxx.com failed: Expected response code 250 but got code "", with message "" src/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php:383

    Generated by: Unknown account Jul 13, 2018 at 1:43 PM

Stack trace

#0 src/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php(281): Swift_Transport_AbstractSmtpTransport->_assertResponseCode('', Array)
#1 src/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/EsmtpTransport.php(270): Swift_Transport_AbstractSmtpTransport->executeCommand('RSET\r\n', Array, Array)
#2 src/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/Esmtp/Auth/LoginAuthenticator.php(46): Swift_Transport_EsmtpTransport->executeCommand('RSET\r\n', Array)
#3 src/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/Esmtp/AuthHandler.php(176): Swift_Transport_Esmtp_Auth_LoginAuthenticator->authenticate(Object(Swift_SmtpTransport), 'postmaster@mg.c...', '5352c44dfcc3f0c...')
#4 src/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/EsmtpTransport.php(332): Swift_Transport_Esmtp_AuthHandler->afterEhlo(Object(Swift_SmtpTransport))
#5 src/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php(118): Swift_Transport_EsmtpTransport->_doHeloCommand()
#6 src/XF/Mail/Mailer.php(276): Swift_Transport_AbstractSmtpTransport->start()
#7 src/XF/Mail/Queue.php(65): XF\Mail\Mailer->send(Object(Swift_Message))
#8 src/XF/Job/MailQueue.php(11): XF\Mail\Queue->run(7.9803709983826)
#9 src/XF/Job/Manager.php(241): XF\Job\MailQueue->run(7.9803709983826)
#10 src/XF/Job/Manager.php(187): XF\Job\Manager->runJobInternal(Array, 7.9803709983826)
#11 src/XF/Job/Manager.php(76): XF\Job\Manager->runJobEntry(Array, 7.9803709983826)
#12 job.php(15): XF\Job\Manager->runQueue(false, 8)
#13 {main}

Request state

array(4) {
  ["url"] => string(8) "/job.php"
  ["referrer"] => string(54) "https://cafesaxophone.com/threads/ooooh-tempted.25104/"
  ["_GET"] => array(0) {
  }
  ["_POST"] => array(0) {
  }
}