For postfix role having gather_facts is mandatory since there are few configs which rely on ansible_fqdn.

Examples

A simple example that doesn't use SASL relaying:

---
- hosts: all
  roles:
    - postfix
  vars:
    postfix_aliases:
    - { user: root, alias: [email protected] }

Provide the relay host name if you want to enable relaying:

---
- hosts: all
  roles:
    - postfix
  vars:
    postfix_aliases:
    - { user: root, alias: [email protected] }
    postfix_relayhost: mail.yourdomain.org

For AWS SES support:

---
- hosts: all
  roles:
    - postfix
  vars:
    postfix_aliases:
    - { user: root, alias: [email protected] }
    postfix_relayhost: email-smtp.us-east-1.amazonaws.com
    postfix_relaytls: true
    # AWS IAM SES credentials (not access key):
    postfix_sasl_user: AKIXXXXXXXXXXXXXXXXX
    postfix_sasl_password: ASDFXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX