I always use the excellent WP Mail Logging plugin by Christian Zoller on my sites to monitor and troubleshoot emails that have been sent out by the WP system.
I just tried to install it on another site and it seems to have disappeared! I finally found this blog entry on Christian’s site:
“The ownership of the WordPress Plugin WP Mail Logging has been transferred to MailPoet. Their plans are to keep the plugin alive and maintain it.”
However this new plugin has WPv5.0 or higher in the requirements.
What else do people use? Anyone interested in forking this and keeping it going for CP?
EDIT: Should also mention that @james has a petition to add mail logging as a core plugin. https://petitions.classicpress.net/posts/190/add-email-logging-feature