Service users and staff members are no longer receive notifications despite that there are no failed tasks in OA and all tasks of
Send notification to ... kind complete successfully.
On sendmail part in
/var/log/maillog the following delivery reports could be seen:
sendmail: v4JM06W5007824: to=<firstname.lastname@example.org>, delay=00:00:00, xdelay=00:00:00, mailer=esmtp, pri=121147, relay=mx1.provider.tld. [18.104.22.168], dsn=5.1.1, stat=User unknown sendmail: v4JM06W5007824: v4JM06W5007826: DSN: User unknown
Operations Automation management node is not able to resolve example.com domain and thus send a message there.
Check that domain of recipient is valid and management node is configured to use valid nameservers.