When working on a development site, or even a production site, you may inadvertently trigger emails that send to real users when testing or making changes.
Ideally, you don’t want any users to know or suspect that you are working on the site, so it’s important to suppress emails that might be triggered by your work. Fortunately, there is a WordPress plugin that will do this for you automatically.
Stop Emails
The Stop Emails plugin, created by Sal Ferrarello, works silently to prevent any outgoing emails from being sent using wp_mail() function. WordPress will continue to act normally, as if the emails were sent successfully.
When the plugin is active, you’ll see a notice in the admin area to remind you that emails are not currently being sent.
I hope this helps.
Source : WPTavern