Watchdog can not send notifications if 127.0.0.1(localhost) is removed from mail server white list.
/var/log/maillog following can be found:
CONFIG_TEXT: Jan 5 10:45:43 hostname.tld postfix/smtpd: NOQUEUE: reject: RCPT from localhost[127.0.0.1]: 454 4.7.1 <[email protected]>: Relay access denied; from=<[email protected]> to=<[email protected]> proto=SMTP helo=<hostname.tld>
This is a bug PPPM-7838 - Watchdog can not send notifications if localhost is removed from white list of mail server, that will be fixed in further Plesk updates.
Currently there is only one workaround is to add localhost to white list of mail server:
Plesk > Tools & Settings > Mail Server Settings > White List > Add Network and add 127.0.0.1