ansible-mailserver-debian/mail_system/tasks/remove_other_mtas.yml

- name: find all installed versions of mail-transport-agent except postfix
  shell: aptitude search '~i~Pmail-transport-agent' | sed -e 's/^...\(.*\)/\1/' | awk '{ print $1 }' | grep -v postfix || /bin/true
  register: mtas

- name: purge MTAs other than postfix
  apt:
    name: "{{ item }}"
    state: absent
    purge: yes
  loop: "{{ mtas.stdout_lines|list }}"