I am running on 15.0.5 (upgraded to 15.0.2). As far as I can tell, both versions had this exact same problem. I have mail configured and test is working as expected. However, if I attempt to share a file/folder to someone external with email, it is failing with the following error logged.
{"reqId":"CMlzSR99R6h7Af04DF7F","level":3,"time":"2019-03-04T06:45:12+00:00","remoteAddr":"10.1.20.10","user":"E991A7B5-73DC-4DE5-9148-6DB5C220C52B","app":"sharebymail","method":"POST","url":"/ocs/v2.php/apps/files_sharing/api/v1/shares?format=json","message":{"Exception":"Swift_TransportException","Message":"Expected response code 354 but got code \"503\", with message \"503 5.5.2 Need rcpt command\r\n\"","Code":503,"Trace":[{"file":"/var/www/nextcloud/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php","line":341,"function":"assertResponseCode","class":"Swift_Transport_AbstractSmtpTransport","type":"->","args":["503 5.5.2 Need rcpt command\r\n",[354]]},{"file":"/var/www/nextcloud/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/EsmtpTransport.php","line":305,"function":"executeCommand","class":"Swift_Transport_AbstractSmtpTransport","type":"->","args":["DATA\r\n",[354],["someone@gmail.com"],false,null]},{"file":"/var/www/nextcloud/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php","line":390,"function":"executeCommand","class":"Swift_Transport_EsmtpTransport","type":"->","args":["DATA\r\n",[354],["someone@gmail.com"]]},{"file":"/var/www/nextcloud/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php","line":497,"function":"doDataCommand","class":"Swift_Transport_AbstractSmtpTransport","type":"->","args":[["someone@gmail.com"]]},{"file":"/var/www/nextcloud/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php","line":516,"function":"doMailTransaction","class":"Swift_Transport_AbstractSmtpTransport","type":"->","args":[{"__class__":"Swift_Message"},"admin@dotslash.io",["someone@gmail.com"],["someone@gmail.com"]]},{"file":"/var/www/nextcloud/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php","line":206,"function":"sendTo","class":"Swift_Transport_AbstractSmtpTransport","type":"->","args":[{"__class__":"Swift_Message"},"admin@dotslash.io",{"someone@gmail.com":null},["someone@gmail.com"]]},{"file":"/var/www/nextcloud/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Mailer.php","line":71,"function":"send","class":"Swift_Transport_AbstractSmtpTransport","type":"->","args":[{"__class__":"Swift_Message"},["someone@gmail.com"]]},{"file":"/var/www/nextcloud/lib/private/Mail/Mailer.php","line":180,"function":"send","class":"Swift_Mailer","type":"->","args":[{"__class__":"Swift_Message"},["someone@gmail.com"]]},{"file":"/var/www/nextcloud/apps/sharebymail/lib/ShareByMailProvider.php","line":442,"function":"send","class":"OC\\Mail\\Mailer","type":"->","args":[{"__class__":"OC\\Mail\\Message"}]},{"file":"/var/www/nextcloud/apps/sharebymail/lib/ShareByMailProvider.php","line":354,"function":"sendMailNotification","class":"OCA\\ShareByMail\\ShareByMailProvider","type":"->","args":["2018-11-06","https://nextcloud.dotslash.io/s/9TF9Tw5iaspZdMq","E991A7B5-73DC-4DE5-9148-6DB5C220C52B","someone@gmail.com",null]},{"file":"/var/www/nextcloud/apps/sharebymail/lib/ShareByMailProvider.php","line":182,"function":"createMailShare","class":"OCA\\ShareByMail\\ShareByMailProvider","type":"->","args":[{"__class__":"OC\\Share20\\Share"}]},{"file":"/var/www/nextcloud/lib/private/Share20/Manager.php","line":670,"function":"create","class":"OCA\\ShareByMail\\ShareByMailProvider","type":"->","args":[{"__class__":"OC\\Share20\\Share"}]},{"file":"/var/www/nextcloud/apps/files_sharing/lib/Controller/ShareAPIController.php","line":567,"function":"createShare","class":"OC\\Share20\\Manager","type":"->","args":[{"__class__":"OC\\Share20\\Share"}]},{"file":"/var/www/nextcloud/lib/private/AppFramework/Http/Dispatcher.php","line":166,"function":"createShare","class":"OCA\\Files_Sharing\\Controller\\ShareAPIController","type":"->","args":[{"__class__":"OC\\Files\\Node\\Folder"},31,4,"someone@gmail.com","false","",null,"",""]},{"file":"/var/www/nextcloud/lib/private/AppFramework/Http/Dispatcher.php","line":99,"function":"executeController","class":"OC\\AppFramework\\Http\\Dispatcher","type":"->","args":[{"__class__":"OCA\\Files_Sharing\\Controller\\ShareAPIController"},"createShare"]},{"file":"/var/www/nextcloud/lib/private/AppFramework/App.php","line":118,"function":"dispatch","class":"OC\\AppFramework\\Http\\Dispatcher","type":"->","args":[{"__class__":"OCA\\Files_Sharing\\Controller\\ShareAPIController"},"createShare"]},{"file":"/var/www/nextcloud/lib/private/AppFramework/Routing/RouteActionHandler.php","line":47,"function":"main","class":"OC\\AppFramework\\App","type":"::","args":["OCA\\Files_Sharing\\Controller\\ShareAPIController","createShare",{"__class__":"OC\\AppFramework\\DependencyInjection\\DIContainer"},{"_route":"ocs.files_sharing.ShareAPI.createShare"}]},{"function":"__invoke","class":"OC\\AppFramework\\Routing\\RouteActionHandler","type":"->","args":[{"_route":"ocs.files_sharing.ShareAPI.createShare"}]},{"file":"/var/www/nextcloud/lib/private/Route/Router.php","line":297,"function":"call_user_func","args":[{"__class__":"OC\\AppFramework\\Routing\\RouteActionHandler"},{"_route":"ocs.files_sharing.ShareAPI.createShare"}]},{"file":"/var/www/nextcloud/ocs/v1.php","line":82,"function":"match","class":"OC\\Route\\Router","type":"->","args":["/ocsapp/apps/files_sharing/api/v1/shares"]},{"file":"/var/www/nextcloud/ocs/v2.php","line":24,"args":["/var/www/nextcloud/ocs/v1.php"],"function":"require_once"}],"File":"/var/www/nextcloud/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php","Line":457,"CustomMessage":"Failed to send share by mail."},"userAgent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/72.0.3626.121 Safari/537.36","version":"15.0.5.3","id":"5c7cc9d723340"}