Error core Error while running background job

Hello! Enroll persistent errors in the. Can anyone met such.

This happened when changing AJAX to cron

Crontab -u www-data -e

  • / 15 * * * * php -f /var/www/owncloud/cron.php

Users receive an electronic message every 20-30 minutes indicating that the following events have occurred. The text is constantly repeated
"Hello, Svetlana Kopets,
You received this email because the following events occurred: http: //x.x.x.x//owncloud/
* Anastasia Bazarevskaya shared with you English - December 14, 2016, 17:56:51
* Aleksey Rai shared with you Plan a schedule for entering new equipment.xlsx - December 17, 2016, 21:23:45 "

Error while running background job (class: OCA\Activity\BackgroundJob\EmailNotification, arguments: ): {"Exception":"Swift_TransportException","Message":"Expected response code 250 but got code \"551\", with message \"551 200104616 message sending quota exceeded, try later\r\n\"","Code":0,"Trace":"#0 \/var\/www\/html\/owncloud\/3rdparty\/swiftmailer\/swiftmailer\/lib\/classes\/Swift\/Transport\/AbstractSmtpTransport.php(281): Swift_Transport_AbstractSmtpTransport->assertResponseCode('551 200104616 m...', Array)\n#1 \/var\/www\/html\/owncloud\/3rdparty\/swiftmailer\/swiftmailer\/lib\/classes\/Swift\/Transport\/EsmtpTransport.php(243): SwiftTransport_AbstractSmtpTransport->executeCommand('\r\n.\r\n', Array, Array)\n#2 \/var\/www\/html\/owncloud\/3rdparty\/swiftmailer\/swiftmailer\/lib\/classes\/Swift\/Transport\/AbstractSmtpTransport.php(333): Swift_Transport_EsmtpTransport->executeCommand('\r\n.\r\n', Array)\n#3 \/var\/www\/html\/owncloud\/3rdparty\/swiftmailer\/swiftmailer\/lib\/classes\/Swift\/Transport\/AbstractSmtpTransport.php(428): Swift_Transport_AbstractSmtpTransport->streamMessage(Object(SwiftMessage))\n#4 \/var\/www\/html\/owncloud\/3rdparty\/swiftmailer\/swiftmailer\/lib\/classes\/Swift\/Transport\/AbstractSmtpTransport.php(444): Swift_Transport_AbstractSmtpTransport->doMailTransaction(Object(SwiftMessage), 'turovcloud@turo...', Array, Array)\n#5 \/var\/www\/html\/owncloud\/3rdparty\/swiftmailer\/swiftmailer\/lib\/classes\/Swift\/Transport\/AbstractSmtpTransport.php(176): Swift_Transport_AbstractSmtpTransport->sendTo(Object(SwiftMessage), 'turovcloud@turo...', Array, Array)\n#6 \/var\/www\/html\/owncloud\/lib\/private\/mail\/mailer.php(105): Swift_Transport_AbstractSmtpTransport->send(Object(Swift_Message), Array)\n#7 \/var\/www\/html\/owncloud\/apps\/activity\/lib\/mailqueuehandler.php(264): OC\Mail\Mailer->send(Object(OC\Mail\Message))\n#8 \/var\/www\/html\/owncloud\/apps\/activity\/lib\/backgroundjob\/emailnotification.php(134): OCA\Activity\MailQueueHandler->sendEmailToUser('4E281FF3-49DA-4...', 'bua@turovmilk.b...', 'ru', 'Asia\/Baghdad', 1496210605)\n#9 \/var\/www\/html\/owncloud\/apps\/activity\/lib\/backgroundjob\/emailnotification.php(94): OCA\Activity\BackgroundJob\EmailNotification->runStep(500, 1496210605)\n#10 \/var\/www\/html\/owncloud\/lib\/private\/backgroundjob\/job.php(52): OCA\Activity\BackgroundJob\EmailNotification->run(NULL)\n#11 \/var\/www\/html\/owncloud\/lib\/private\/backgroundjob\/timedjob.php(53): OC\BackgroundJob\Job->execute(Object(OC\BackgroundJob\JobList), Object(OC\Log))\n#12 \/var\/www\/html\/owncloud\/cron.php(145): OC\BackgroundJob\TimedJob->execute(Object(OC\BackgroundJob\JobList), Object(OC\Log))\n#13 {main}","File":"\/var\/www\/html\/owncloud\/3rdparty\/swiftmailer\/swiftmailer\/lib\/classes\/Swift\/Transport\/AbstractSmtpTransport.php","Line":383}

Server configuration
"user_ldap" 0.8.0
Operating system: Ubuntu Server 15.10
Web server:Apache/2.4.12
Database:mariaDB 10.0.25
PHP version:PHP 5.6.11
ownCloud version (see ownCloud admin page): ownCloud 9.0.9 (stable)
Updated from an older ownCloud or fresh install:
Special configuration (external storage - nas, external authentication- ldap, reverse proxy - no, server-side-encryption-no):

Probably just the queue of messages which got stuck while using AJAX which now get sent out after switching to cron.

How to clear a message queue?