Swift_TransportException: Expected response code 354 but got an empty response

Nextcloud version: 16.0.5
Operating system and version: psa v17.8.11, build1708180301.19 // CentOS 7
Apache or nginx version: nginx, version unknown
PHP version: 7.3.9

The issue you are facing:
Our nextcloud is hosted on an external webserver with plesk onyx interface.
E-Mails form nextcloud are sent to our internal domain without any problem.
SMTP settings seem to be correct, testmails are delivered as they should be,
as do password-reset and welcome mails into our internal domain.
However, if a welcome or password reset mail is to be sent to an external user, he wont receive it.
The log in nextcloud shows sais:
“Swift_TransportException: Expected response code 354 but got an empty response”

I googled that error and found that some users face it in general, so that means that SMTP-settings in nextcloud are not correct. But mails into our own internal domain work perfectly.
The issue just occurs for mails sendt to other domains.

Is this the first time you’ve seen this error? (Y/N): Y

The output of your Nextcloud log in Admin > Logging:

	Swift_TransportException: Expected response code 354 but got an empty response

    /var/www/vhosts/<domain>.com/cloud.<domain>.com/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php - line 341:

    Swift_Transport_AbstractSmtpTransport->assertResponseCode("", [ 354])

    /var/www/vhosts/<domain>.com/cloud.<domain>.com/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/EsmtpTransport.php - line 305:

    Swift_Transport_AbstractSmtpTransport->executeCommand("DATA\r\n", [ 354], [ "receipient@domain.com"], false, null)

    /var/www/vhosts/<domain>.com/cloud.<domain>.com/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php - line 390:

    Swift_Transport_EsmtpTransport->executeCommand("DATA\r\n", [ 354], [ "receipient@domain.com"])

    /var/www/vhosts/<domain>.com/cloud.<domain>.com/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php - line 497:

    Swift_Transport_AbstractSmtpTransport->doDataCommand([ "receipient@domain.com"])

    /var/www/vhosts/<domain>.com/cloud.<domain>.com/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php - line 516:

    Swift_Transport_AbstractSmtpTransport->doMailTransaction(Swift_Message {}, "cloud@<domain>.com", [ "receipient@domain.com"], [ "receipient@domain.com"])

    /var/www/vhosts/<domain>.com/cloud.<domain>.com/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php - line 206:

    Swift_Transport_AbstractSmtpTransport->sendTo(Swift_Message {}, "cloud@<domain>.com", { commercial ... "}, [ "receipient@domain.com"])

    /var/www/vhosts/<domain>.com/cloud.<domain>.com/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Mailer.php - line 71:

    Swift_Transport_AbstractSmtpTransport->send(Swift_Message {}, [ "receipient@domain.com"])

    /var/www/vhosts/<domain>.com/cloud.<domain>.com/lib/private/Mail/Mailer.php - line 180:

    Swift_Mailer->send(Swift_Message {}, [ "receipient@domain.com"])

    /var/www/vhosts/onskinery.com/cloud.onskinery.com/settings/Mailer/NewUserMailHelper.php - line 169:

    OC\Mail\Mailer->send(OC\Mail\Message {})

    /var/www/vhosts/onskinery.com/cloud.onskinery.com/apps/provisioning_api/lib/Controller/UsersController.php - line 909:

    OC\Settings\Mailer\NewUserMailHelper->sendMail(OC\User\User {}, OC\Mail\EMailTemplate {})

    /var/www/vhosts/onskinery.com/cloud.onskinery.com/lib/private/AppFramework/Http/Dispatcher.php - line 166:

    OCA\Provisioning_API\Controller\UsersController->resendWelcomeMessage("<username>")

    /var/www/vhosts/onskinery.com/cloud.onskinery.com/lib/private/AppFramework/Http/Dispatcher.php - line 99:

    OC\AppFramework\Http\Dispatcher->executeController(OCA\Provisio ... {}, "resendWelcomeMessage")

    /var/www/vhosts/onskinery.com/cloud.onskinery.com/lib/private/AppFramework/App.php - line 126:

    OC\AppFramework\Http\Dispatcher->dispatch(OCA\Provisio ... {}, "resendWelcomeMessage")

    /var/www/vhosts/onskinery.com/cloud.onskinery.com/lib/private/AppFramework/Routing/RouteActionHandler.php - line 47:

    OC\AppFramework\App::main("OCA\\Provis ... r", "resendWelcomeMessage", OC\AppFramew ... {}, { userId: "f ... "})

    <<closure>>

    OC\AppFramework\Routing\RouteActionHandler->__invoke({ userId: "f ... "})

    /var/www/vhosts/onskinery.com/cloud.onskinery.com/lib/private/Route/Router.php - line 297:

    call_user_func(OC\AppFramew ... {}, { userId: "f ... "})

    /var/www/vhosts/onskinery.com/cloud.onskinery.com/ocs/v1.php - line 82:

    OC\Route\Router->match("/ocsapp/clo ... e")

    /var/www/vhosts/onskinery.com/cloud.onskinery.com/ocs/v2.php - line 24:

    require_once("/var/www/vh ... p")

The output of your Apache/nginx/system log in /var/log/____:

2019-10-01 09:53:35	Error	<ext.IP>		AH01068: Got bogus version 118				Apache-Fehler
2019-10-01 09:53:35	Error	<ext.IP>		(22)Invalid argument: AH01075: Error dispatching request to :				Apache-Fehler
2019-10-01 09:54:50	Error	<ext.IP>		AH01070: Error parsing script headers				Apache-Fehler
2019-10-01 09:56:52	Error	<ext.IP>		AH01070: Error parsing script headers				Apache-Fehler
2019-10-01 09:59:50	Error	<ext.IP>		AH01070: Error parsing script headers				Apache-Fehler
2019-10-01 10:04:50	Error	<ext.IP>		AH01070: Error parsing script headers				Apache-Fehler
2019-10-01 10:09:50	Error	<ext.IP>		AH01070: Error parsing script headers				Apache-Fehler
2019-10-01 10:14:50	Error	<ext.IP>		AH01070: Error parsing script headers				Apache-Fehler
2019-10-01 10:19:49	Error	<ext.IP>		AH01070: Error parsing script headers				Apache-Fehler
2019-10-01 10:24:50	Error	<ext.IP>		AH01070: Error parsing script headers				Apache-Fehler
2019-10-01 10:29:50	Error	<ext.IP>		AH01070: Error parsing script headers				Apache-Fehler
2019-10-01 10:30:00	Error	<ext.IP>		AH01070: Error parsing script headers				Apache-Fehler
2019-10-01 10:30:08	Error	<ext.IP>		AH01070: Error parsing script headers

Problem solved.

Had to adjust SMTP Settings according to the Mail Relay that is configured in our Firewall.
Now it works.