Thanks for your help, here the full log.
AFAIK the SMTP server only accepts TLS1.0…
My server is Ubuntu 20.04, PHP 7.4 + WordPress 5.5, OpenSSL 1.1.1f
Versions:
WordPress: 5.5
WordPress MS: Yes
PHP: 7.4.3
WP Mail SMTP: 2.5.1
Params:
Mailer: smtp
Constants: No
ErrorInfo: SMTP Error: Could not connect to SMTP host.
Host: mail.postassl.it
Port: 587
SMTPSecure: tls
SMTPAutoTLS: bool(true)
SMTPAuth: bool(true)
SMTPOptions: {"ssl":{"verify_peer":false,"verify_peer_name":false,"allow_self_signed":true}}
Server:
OpenSSL: OpenSSL 1.1.1f 31 Mar 2020
Debug:
Mailer: Other SMTP
SMTP Error: Could not connect to SMTP host.
SMTP Debug:
2020-12-24 09:20:36 Connection: opening to mail.postassl.it:587, timeout=300, options=array ( 'ssl' => array ( 'verify_peer' => false, 'verify_peer_name' => false, 'allow_self_signed' => true, ),)
2020-12-24 09:20:36 Connection: opened
2020-12-24 09:20:36 SMTP INBOUND: "220 smtpcm1-pc.aruba.it bizsmtp ESMTP server ready"
2020-12-24 09:20:36 SERVER -> CLIENT: 220 smtpcm1-pc.aruba.it bizsmtp ESMTP server ready
2020-12-24 09:20:36 CLIENT -> SERVER: EHLO hosting.pa-online.it
2020-12-24 09:20:36 SMTP INBOUND: "250-smtpcm1-pc.aruba.it hello [34.91.190.87], pleased to meet you"
2020-12-24 09:20:36 SMTP INBOUND: "250-HELP"
2020-12-24 09:20:36 SMTP INBOUND: "250-AUTH LOGIN PLAIN"
2020-12-24 09:20:36 SMTP INBOUND: "250-SIZE 524288000"
2020-12-24 09:20:36 SMTP INBOUND: "250-ENHANCEDSTATUSCODES"
2020-12-24 09:20:36 SMTP INBOUND: "250-8BITMIME"
2020-12-24 09:20:36 SMTP INBOUND: "250-STARTTLS"
2020-12-24 09:20:36 SMTP INBOUND: "250 OK"
2020-12-24 09:20:36 SERVER -> CLIENT: 250-smtpcm1-pc.aruba.it hello [34.91.190.87], pleased to meet you250-HELP250-AUTH LOGIN PLAIN250-SIZE 524288000250-ENHANCEDSTATUSCODES250-8BITMIME250-STARTTLS250 OK
2020-12-24 09:20:36 CLIENT -> SERVER: STARTTLS
2020-12-24 09:20:36 SMTP INBOUND: "220 2.0.0 Ready to start TLS"
2020-12-24 09:20:36 SERVER -> CLIENT: 220 2.0.0 Ready to start TLS
2020-12-24 09:20:36 Connection failed. Error #2: stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages:error:1425F102:SSL routines:ssl_choose_client_version:unsupported protocol [/var/www/webpa/wp-includes/PHPMailer/SMTP.php line 426]
SMTP Error: Could not connect to SMTP host.
2020-12-24 09:20:36 CLIENT -> SERVER: QUIT