When sharing a password protected link to an external user the external user receives the link.
When sharing a password protected link to an internal user they never receive an email notice stating that the file was shared.
Certainties: The Nextcloud server and the internal email server on currently on the same domain. We will be moving the Nextcloud server to a DMZ after configuration is complete.
We are using sendmail.
The internal email server is Exchange 2016 and has a mal-configured SSL cert.