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

- name: Find all installed versions of mail-transport-agent except postfix
  ansible.builtin.shell:
    cmd: set -o pipefail && aptitude search '~i~Pmail-transport-agent' | sed -e 's/^...\(.*\)/\1/' | awk '{ print $1 }' | grep -v postfix || /bin/true
    executable: /bin/bash
  register: mtas
  changed_when: false

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