Subject

I would like to contribute to the project

Description

We need a follow-up of #3732. Tasks:

•  use .txt suffix for _send_email and _nc_wrapper
•  in smtp_delivery.bats, remove the need for existing/userX and remove these files (ref: https://github.com/docker-mailserver/docker-mailserver/pull/3732/files#r1438975373)
•  add a new helper _send_email_unchecked that works like _send_email but has no assert_success directly afterward to check that all went well and add such an assert_success in _send_email
• use --auth PLAIN instead --auth LOGIN ?