E-mails sent from OBAS are not being delivered.
In /var/log/maillog there are messages like
Oct 23 12:37:47 vps sendmail: s9N4blZm026377: to==?UTF-8?B?R2VuZSBUYW4gRWkgU3pl?=<firstname.lastname@example.org>, ctladdr=apache (48/48), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=41118, relay=[127.0.0.1] [127.0.0.1], dsn=5.0.0, stat=Service unavailable
One of possible reasons is incorrect DNS servers configuration.
Please check if the recipient mail domain can be resolved from OBAS server.