Increase visibility of e-mail delivery failures

⚠️ This issue respects the following points: ⚠️ • This is a bug, not a question or a configuration/webserver/proxy issue. • This issue is not already reported on [Github]( OR [Nextcloud Community Forum]( _(I've searched it)_. • Nextcloud Server is up to date. See [Maintenance and Release Schedule]( for supported versions. • I agree to follow Nextcloud's [Code of Conduct]( Bug description When there is a configuration error or the mail server is unable to accept mails from nextcloud (e.g. due to fail2ban) there is no indication that nextcloud is not able to send emails in the nextcloud logging or in the warnings on admin page. Steps to reproduce 1. Block the server's ip adress on which nextcloud is installed on your mail server 2. Try to send a mail through nextcloud (e.g. when sharing something to an email adress. 3. No email is sent, but there is no error to the user, in the logs or in the warnings on admin page. Expecte…