Since upgrading a server from 15.7 to 100.0.7053 I am now seeing emails from the "System Administrator" being sent to remote addresses that are bouncing.
Failure messages like this that were not previously occurring are showing up in the Delivery Log:
[2019.06.05] 06:03:29.477  Delivery for System Administrator to email@example.com has bounced. Reason: Remote host said: 501 5.1.7 Invalid address [SN1NAM01FT009.eop-nam01.prod.protection.outlook.com]
[2019.06.05] 07:58:21.077  Delivery for System Administrator to firstname.lastname@example.org has bounced. Reason: Remote host said: 555 MAIL FROM syntax error (#5.5.2)
[2019.06.05] 07:53:19.186  Delivery for System Administrator to email@example.com has bounced. Reason: Remote host said: 501 5.1.7 Bad sender address syntax
They always follow a successful delivery from the domain to an internal address, so it would seem to be a Delivery Receipt or some such going back.
I started adding valid email address to the System Messages, but so far it hasn't changed.
Do I need to restart the service?
Again, these bounces were definitely not happening prior to the upgrade.