Flector

The plugin sends you an email each time a comment goes to the spam folder. The plugin will come in handy in combination with anti-spam plugins like <a href="https://wordpress.org/plugins/antispam-bee/" rel="ugc">Antispam Bee</a>, <a href="https://wordpress.org/plugins/invisible-captcha/" rel="ugc">Invisible Captcha</a> and others that completely filter out comments from bots. It saves you the trouble of having to visit the “Spam” folder and manually check whether any real comments have been marked as spam by mistake.
We don’t recommend installing this plugin if you use <a href="https://wordpress.org/plugins/akismet/" rel="ugc">Akismet</a> only – you’ll be drowned in hundreds of emails every day.
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Spam Notifier

The plugin sends an email message when a comment goes to the spam folder.

bgribaudo

Attempts to prevent automated SPAM by requiring visitors to answer a question of your choosing in order for their comment to be accepted. Presumably, spambots will be unable to correctly answer this question and so comments they attempt to post will be rejected.
Only regular comments from non-logged-in users are filtered. Pingbacks, trackbacks and authenticated users will not be prompted to answer the verification question.
Privacy Note: This plugin does not call home or communicate with any external servers or services. All processing is done locally within your WordPress site.

Super Simple SPAM Stopper

Prevents automated SPAM by requiring visitors to answer a question of your choosing (plaintext or regex).

Spam Notifier vs Super Simple SPAM Stopper