rainloop is part of nextcloud. If it tries to speak to some SMTP server, it uses a PHP smtp sub-module, which does AUTH PLAIN with multiline (!), which fails if speaking to exim.
and this comment in this very long issue:
needs to be changed:
public $__USE_SINGLE_LINE_AUTH_PLAIN_COMMAND = false;
public $__USE_SINGLE_LINE_AUTH_PLAIN_COMMAND = true;
Makes sense to chime in on the Rainloop issue, but also with the Nextcloud app on https://github.com/pierre-alain-b/rainloop-nextcloud/issues
Modifying the app will lead to signature errors with the Nextcloud checks.