What could be causing this error?
Edit:
It may be due to the extension https://discuss.flarum.org/d/4762-friendsofflarum-by-bu-well-integrated-advanced-private-discussions/ . I am getting an error when responding to private messages.
[2020-08-20 09:13:10] production.ERROR: Swift_TransportException: Expected response code 250 but got code "554", with message "554 5.7.1 [2] Message rejected under suspicion of SPAM; https://ya.cc/1IrBc 1597914790-q236Hrnhc8-D9nSfqn3
" in /var/www/sourceturk/flarum/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php:457
Stack trace:
#0 /var/www/sourceturk/flarum/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php(341): Swift_Transport_AbstractSmtpTransport->assertResponseCode()
#1 /var/www/sourceturk/flarum/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/EsmtpTransport.php(305): Swift_Transport_AbstractSmtpTransport->executeCommand()
#2 /var/www/sourceturk/flarum/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php(404): Swift_Transport_EsmtpTransport->executeCommand()
#3 /var/www/sourceturk/flarum/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php(500): Swift_Transport_AbstractSmtpTransport->streamMessage()
#4 /var/www/sourceturk/flarum/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php(516): Swift_Transport_AbstractSmtpTransport->doMailTransaction()
#5 /var/www/sourceturk/flarum/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php(206): Swift_Transport_AbstractSmtpTransport->sendTo()
#6 /var/www/sourceturk/flarum/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Mailer.php(71): Swift_Transport_AbstractSmtpTransport->send()
#7 /var/www/sourceturk/flarum/vendor/illuminate/mail/Mailer.php(483): Swift_Mailer->send()
#8 /var/www/sourceturk/flarum/vendor/illuminate/mail/Mailer.php(259): Illuminate\Mail\Mailer->sendSwiftMessage()
#9 /var/www/sourceturk/flarum/vendor/flarum/core/src/Notification/NotificationMailer.php(43): Illuminate\Mail\Mailer->send()
#10 /var/www/sourceturk/flarum/vendor/flarum/core/src/Notification/Job/SendEmailNotificationJob.php(37): Flarum\Notification\NotificationMailer->send()
#11 [internal function]: Flarum\Notification\Job\SendEmailNotificationJob->handle()
#12 /var/www/sourceturk/flarum/vendor/illuminate/container/BoundMethod.php(29): call_user_func_array()
#13 /var/www/sourceturk/flarum/vendor/illuminate/container/BoundMethod.php(87): Illuminate\Container\BoundMethod::Illuminate\Container\{closure}()
#14 /var/www/sourceturk/flarum/vendor/illuminate/container/BoundMethod.php(31): Illuminate\Container\BoundMethod::callBoundMethod()
#15 /var/www/sourceturk/flarum/vendor/illuminate/container/Container.php(572): Illuminate\Container\BoundMethod::call()
#16 /var/www/sourceturk/flarum/vendor/illuminate/bus/Dispatcher.php(94): Illuminate\Container\Container->call()
#17 /var/www/sourceturk/flarum/vendor/illuminate/pipeline/Pipeline.php(128): Illuminate\Bus\Dispatcher->Illuminate\Bus\{closure}()
#18 /var/www/sourceturk/flarum/vendor/illuminate/pipeline/Pipeline.php(104): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#19 /var/www/sourceturk/flarum/vendor/illuminate/bus/Dispatcher.php(98): Illuminate\Pipeline\Pipeline->then()
#20 /var/www/sourceturk/flarum/vendor/illuminate/queue/CallQueuedHandler.php(49): Illuminate\Bus\Dispatcher->dispatchNow()
#21 /var/www/sourceturk/flarum/vendor/illuminate/queue/Jobs/Job.php(83): Illuminate\Queue\CallQueuedHandler->call()
#22 /var/www/sourceturk/flarum/vendor/illuminate/queue/SyncQueue.php(42): Illuminate\Queue\Jobs\Job->fire()
#23 /var/www/sourceturk/flarum/vendor/flarum/core/src/Notification/NotificationSyncer.php(163): Illuminate\Queue\SyncQueue->push()
#24 /var/www/sourceturk/flarum/vendor/flarum/core/src/Notification/NotificationSyncer.php(110): Flarum\Notification\NotificationSyncer->mailNotifications()
#25 /var/www/sourceturk/flarum/vendor/fof/byobu/src/Jobs/SendNotificationWhenPostedInPrivateDiscussion.php(65): Flarum\Notification\NotificationSyncer->sync()
#26 [internal function]: FoF\Byobu\Jobs\SendNotificationWhenPostedInPrivateDiscussion->handle()
#27 /var/www/sourceturk/flarum/vendor/illuminate/container/BoundMethod.php(29): call_user_func_array()
#28 /var/www/sourceturk/flarum/vendor/illuminate/container/BoundMethod.php(87): Illuminate\Container\BoundMethod::Illuminate\Container\{closure}()
#29 /var/www/sourceturk/flarum/vendor/illuminate/container/BoundMethod.php(31): Illuminate\Container\BoundMethod::callBoundMethod()
#30 /var/www/sourceturk/flarum/vendor/illuminate/container/Container.php(572): Illuminate\Container\BoundMethod::call()
#31 /var/www/sourceturk/flarum/vendor/illuminate/bus/Dispatcher.php(94): Illuminate\Container\Container->call()
#32 /var/www/sourceturk/flarum/vendor/illuminate/pipeline/Pipeline.php(128): Illuminate\Bus\Dispatcher->Illuminate\Bus\{closure}()
#33 /var/www/sourceturk/flarum/vendor/illuminate/pipeline/Pipeline.php(104): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#34 /var/www/sourceturk/flarum/vendor/illuminate/bus/Dispatcher.php(98): Illuminate\Pipeline\Pipeline->then()
#35 /var/www/sourceturk/flarum/vendor/illuminate/queue/CallQueuedHandler.php(49): Illuminate\Bus\Dispatcher->dispatchNow()
#36 /var/www/sourceturk/flarum/vendor/illuminate/queue/Jobs/Job.php(83): Illuminate\Queue\CallQueuedHandler->call()
#37 /var/www/sourceturk/flarum/vendor/illuminate/queue/SyncQueue.php(42): Illuminate\Queue\Jobs\Job->fire()
#38 /var/www/sourceturk/flarum/vendor/fof/byobu/src/Listeners/QueueNotificationJobs.php(59): Illuminate\Queue\SyncQueue->push()
#39 /var/www/sourceturk/flarum/vendor/flarum/core/src/Database/AbstractModel.php(65): FoF\Byobu\Listeners\QueueNotificationJobs->FoF\Byobu\Listeners\{closure}()
#40 /var/www/sourceturk/flarum/vendor/illuminate/events/Dispatcher.php(360): Flarum\Database\AbstractModel::Flarum\Database\{closure}()
#41 /var/www/sourceturk/flarum/vendor/illuminate/events/Dispatcher.php(209): Illuminate\Events\Dispatcher->Illuminate\Events\{closure}()
#42 /var/www/sourceturk/flarum/vendor/illuminate/database/Eloquent/Concerns/HasEvents.php(162): Illuminate\Events\Dispatcher->dispatch()
#43 /var/www/sourceturk/flarum/vendor/illuminate/database/Eloquent/Model.php(692): Illuminate\Database\Eloquent\Model->fireModelEvent()
#44 /var/www/sourceturk/flarum/vendor/illuminate/database/Eloquent/Model.php(663): Illuminate\Database\Eloquent\Model->finishSave()
#45 /var/www/sourceturk/flarum/vendor/flarum/core/src/Post/Command/EditPostHandler.php(86): Illuminate\Database\Eloquent\Model->save()
#46 /var/www/sourceturk/flarum/vendor/illuminate/bus/Dispatcher.php(90): Flarum\Post\Command\EditPostHandler->handle()
#47 /var/www/sourceturk/flarum/vendor/illuminate/pipeline/Pipeline.php(128): Illuminate\Bus\Dispatcher->Illuminate\Bus\{closure}()
#48 /var/www/sourceturk/flarum/vendor/illuminate/pipeline/Pipeline.php(104): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}()
#49 /var/www/sourceturk/flarum/vendor/illuminate/bus/Dispatcher.php(98): Illuminate\Pipeline\Pipeline->then()
#50 /var/www/sourceturk/flarum/vendor/illuminate/bus/Dispatcher.php(76): Illuminate\Bus\Dispatcher->dispatchNow()
#51 /var/www/sourceturk/flarum/vendor/flarum/core/src/Api/Controller/UpdatePostController.php(57): Illuminate\Bus\Dispatcher->dispatch()
#52 /var/www/sourceturk/flarum/vendor/flarum/core/src/Api/Controller/AbstractSerializeController.php(96): Flarum\Api\Controller\UpdatePostController->data()
#53 /var/www/sourceturk/flarum/vendor/flarum/core/src/Http/RouteHandlerFactory.php(38): Flarum\Api\Controller\AbstractSerializeController->handle()
#54 /var/www/sourceturk/flarum/vendor/flarum/core/src/Http/Middleware/DispatchRoute.php(65): Flarum\Http\RouteHandlerFactory->Flarum\Http\{closure}()
#55 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\DispatchRoute->process()
#56 /var/www/sourceturk/flarum/vendor/kilowhat/flarum-ext-audit-free/src/Middlewares/SetLoggerActor.php(28): Laminas\Stratigility\Next->handle()
#57 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/Next.php(61): Kilowhat\Audit\Middlewares\SetLoggerActor->process()
#58 /var/www/sourceturk/flarum/vendor/flarum/core/src/Http/Middleware/SetLocale.php(50): Laminas\Stratigility\Next->handle()
#59 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\SetLocale->process()
#60 /var/www/sourceturk/flarum/vendor/flarum/core/src/Http/Middleware/CheckCsrfToken.php(45): Laminas\Stratigility\Next->handle()
#61 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\CheckCsrfToken->process()
#62 /var/www/sourceturk/flarum/vendor/flarum/core/src/Http/Middleware/AuthenticateWithHeader.php(55): Laminas\Stratigility\Next->handle()
#63 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\AuthenticateWithHeader->process()
#64 /var/www/sourceturk/flarum/vendor/flarum/core/src/Http/Middleware/AuthenticateWithSession.php(32): Laminas\Stratigility\Next->handle()
#65 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\AuthenticateWithSession->process()
#66 /var/www/sourceturk/flarum/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php(51): Laminas\Stratigility\Next->handle()
#67 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\RememberFromCookie->process()
#68 /var/www/sourceturk/flarum/vendor/flarum/core/src/Http/Middleware/StartSession.php(61): Laminas\Stratigility\Next->handle()
#69 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\StartSession->process()
#70 /var/www/sourceturk/flarum/vendor/flarum/core/src/Api/Middleware/FakeHttpMethods.php(29): Laminas\Stratigility\Next->handle()
#71 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Api\Middleware\FakeHttpMethods->process()
#72 /var/www/sourceturk/flarum/vendor/flarum/core/src/Http/Middleware/ParseJsonBody.php(28): Laminas\Stratigility\Next->handle()
#73 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\ParseJsonBody->process()
#74 /var/www/sourceturk/flarum/vendor/flarum/core/src/Http/Middleware/HandleErrors.php(57): Laminas\Stratigility\Next->handle()
#75 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\HandleErrors->process()
#76 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(84): Laminas\Stratigility\Next->handle()
#77 /var/www/sourceturk/flarum/vendor/middlewares/request-handler/src/RequestHandler.php(84): Laminas\Stratigility\MiddlewarePipe->process()
#78 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/Next.php(61): Middlewares\RequestHandler->process()
#79 /var/www/sourceturk/flarum/vendor/middlewares/base-path-router/src/BasePathRouter.php(97): Laminas\Stratigility\Next->handle()
#80 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/Next.php(61): Middlewares\BasePathRouter->process()
#81 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/Middleware/OriginalMessages.php(42): Laminas\Stratigility\Next->handle()
#82 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/Next.php(61): Laminas\Stratigility\Middleware\OriginalMessages->process()
#83 /var/www/sourceturk/flarum/vendor/middlewares/base-path/src/BasePath.php(53): Laminas\Stratigility\Next->handle()
#84 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/Next.php(61): Middlewares\BasePath->process()
#85 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(84): Laminas\Stratigility\Next->handle()
#86 /var/www/sourceturk/flarum/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(73): Laminas\Stratigility\MiddlewarePipe->process()
#87 /var/www/sourceturk/flarum/vendor/laminas/laminas-httphandlerrunner/src/RequestHandlerRunner.php(96): Laminas\Stratigility\MiddlewarePipe->handle()
#88 /var/www/sourceturk/flarum/vendor/flarum/core/src/Http/Server.php(42): Laminas\HttpHandlerRunner\RequestHandlerRunner->run()
#89 /var/www/sourceturk/flarum/public/index.php(26): Flarum\Http\Server->listen()
#90 {main}
