Alertmanager fails to send SMTP notifications due to "starttls failed" errors regarding certificate hostname validity

Issue

• Alertmanager fails to send SMTP notifications for custom alerts.

• The following errors stream in the alertmanager container logs:

  $ oc -n openshift-monitoring logs -f alertmanager-main-0 -c alertmanager
  [...]
  2019-06-11T09:51:00.326870516Z level=error ts=2019-06-11T09:51:00.326553215Z caller=notify.go:332 component=dispatcher msg="Error on notify" err="starttls failed: x509: certificate is valid for smtp.example.com, not smtp-relay.example.com"
  2019-06-11T09:51:00.326900117Z level=error ts=2019-06-11T09:51:00.326668215Z caller=dispatch.go:280 component=dispatcher msg="Notify for alerts failed" num_alerts=1 err="starttls failed: x509: certificate is valid for smtp.example.com, not smtp-relay.example.com"