Configuration of Flag notification Email sender addresses and names

Issue

Flag content warning (e.g. when a user reports a thread on a Message Board as inappropriate) is not sent to admins despite changing the email sender name and address under Control Panel > Instance settings > Email. At the same time log shows " Skipping email because the sender is not specified" warning.

Flag notification email sender addresses and names are implemented separately in FlagsGroupServiceConfiguration.
This means that they need to be configured in System Settings > Community Tools > Flags in the Email from Address and Email from Name fields.
The default values of these fields are ${server-property://com.liferay.portal/admin.email.from.address} and ${server-property://com.liferay.portal/admin.email.from.name}, respectively. Those are server scoped values, not instance scoped, as they are server-properties, so what has been set instance-wide will not have an effect on them.

If you change ${server-property://com.liferay.portal/admin.email.from.address} and ${server-property://com.liferay.portal/admin.email.from.name} to a valid email address with a corresponding name, respectively, and save the configuration, the flag notifications will be sent from that address.