This makes it explicit that outgoing traffic (25/587) needs to be let through, as well as documenting how to debug other non-delivery issues.
Prerequisites
Configuring your DNS server
Configuring this Ansible playbook
Installing
Registering users
Configuring service discovery via .well-known
Maintenance / checking if services work
Maintenance / upgrading services
Maintenance / upgrading PostgreSQL
Maintenance and Troubleshooting
Uninstalling