I am the author of the email-log WordPress plugin, which allows you to log emails sent thorugh WordPress. In order to do that, I listen to the
wp_mail filter in WordPress.
A user of my plugin recently complained that if an email is sent through wpmandrill plugin then it is not getting logged. I looked into it and found that the wpmandrill plugin is changing the names of the array elements that are passed to the
In https://core.trac.wordpress.org/browser/tags/3.9/src/wp-includes/pluggable.php#L236 you can see that the original array key is "message". But in https://plugins.trac.wordpress.org/browser/wpmandrill/trunk/wpmandrill.php#L1646 it is changed to "html".
The wpmandrill plugin should not change the array keys of args passed through filters. It will nice if it can be changed back to "message" in wpmandrill plugin, so that it doesn't break other plugins.