Nextcloud version 14.0.3
Apache 2434.7210
PHP version 7.2.10
An error occurs when sending e-mail “A problem occurred while sending the email. Please revise your settings. (Error: Expected response code 220 but got code “”, with message “”)”
What can it be related to?
how do you send your mails? php?
No, I have “sendmail” option in settings.
try smtp with an external provider. i know that phpmail got deceased in version 14…
and there was something with sendmail as well (i can’t clearly remember - and am too short in time to search for it)
BTW, we are no God. I wonder who can guess what you have done and among which what is wrong. 
In SMTP mode I have apache error 
Error PHP stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed at /share/CACHEDEV1_DATA/.qpkg/NextCloud/nextcloud/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/StreamBuffer.php#94 2018-10-17T12:12:59+0200 Error PHP stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed at /share/CACHEDEV1_DATA/.qpkg/NextCloud/nextcloud/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/StreamBuffer.php#94 2018-10-17T12:12:54+0200 Error PHP stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed at /share/CACHEDEV1_DATA/.qpkg/NextCloud/nextcloud/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/StreamBuffer.php#94 2018-10-17T12:12:48+0200 Error PHP stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed at /share/CACHEDEV1_DATA/.qpkg/NextCloud/nextcloud/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/StreamBuffer.php#94 2018-10-17T12:12:45+0200 Error PHP stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed at /share/CACHEDEV1_DATA/.qpkg/NextCloud/nextcloud/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/StreamBuffer.php#94 2018-10-17T12:12:29+0200 Error PHP stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed at /share/CACHEDEV1_DATA/.qpkg/NextCloud/nextcloud/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/StreamBuffer.php#94 2018-10-17T12:12:22+0200 Error PHP stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed at /share/CACHEDEV1_DATA/.qpkg/NextCloud/nextcloud/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/StreamBuffer.php#94 2018-10-17T12:11:20+0200 Error PHP stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed at /share/CACHEDEV1_DATA/.qpkg/NextCloud/nextcloud/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/StreamBuffer.php#94 2018-10-17T11:48:37+0200 Error PHP stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed at /share/CACHEDEV1_DATA/.qpkg/NextCloud/nextcloud/3rdparty/swiftmailer/swiftmailer/lib/classes/Swift/Transport/StreamBuffer.php#94
without digging any deeper into it: it looks like a certificate problem. what kind of certificate do you have installed? self-signed?
and… pls search for TLS problems and swiftmail - there has been an issue (of which i thought it was solved some time ago) so maybe this would get you a bit closer into the solution