Qmail is selected as SMTP server
All mail clients throw the error with the inability to connect to SMTP, Roundcube shows:
SMTP Error (-1): Connection to server failed
The SSL It! extension is installed
The TLS versions and ciphers management option is set to Modern
There are such errors in the /var/log/maillog file:
websrv04 /var/qmail/bin/relaylock: /var/qmail/bin/relaylock: mail from 203.0.113.2:52396 (not defined)
websrv04 /var/qmail/bin/relaylock: /var/qmail/bin/relaylock: mail from 0000:0000:0000:0000:0000:0000:0000:0000:42884 (not defined)
If Roundcube is used there are such errors in the /var/log/plesk-roundcube/errors file:
ERROR: stream_socket_client(): unable to connect to ssl://localhost:465 (Unknown error) (0)
ERROR: Failed to connect socket: stream_socket_client(): unable to connect to ssl://localhost:465 (Unknown error) ()
SMTP Error: Connection failed: Failed to connect socket: stream_socket_client(): unable to connect to ssl://localhost:465 (Unknown error) in /usr/share/psa-roundcube/program/lib/Roundcube/rcube.php on line 1667 (POST /?_task=mail&_unlock=loading1583560215548&_lang=en_US&_framed=1&_action=send)
If Horde is used there are such errors in the file:
ERR: HORDE [imp] Error connecting to SMTP server.  [pid 27450 on line 1160 of "/usr/share/psa-horde/imp/lib/Compose.php"]
Qmail does not support modern TLS ciphers.
To find possible workaround there was created bug #PPP-47997: Mail does not work with Qmail and enabled Modern TLS ciphers.
There are two possible workarounds.
Set lower TLS cipher version
- Log into Plesk
- Go to Tools & Settings > TLS versions and ciphers management
- Set the TLS versions and ciphers by Mozilla option to Intermediate or Old
Change Qmail to Postfix
Switch from Qmail to Postfix as explained here: How to install/change a local mail server in Plesk for Linux