The MailCatcher helper allows you to view all outgoing mail from development instances only. This is especially useful if making changes to email templates etc.
All emails will be caught by MailCatcher, regardless of the from or to address. As long as it is being sent by the instance it will be caught.
(If you don't see emails show up in MailCatcher, you may have a SMTP Magento module installed, which will bypass core mail sending processes. Please disable any SMTP / 3rd party email management related extensions).
Caught emails don't go any further, so they won't show up in anyone's actual email box.
To use MailCatcher, you first need to set 'Outgoing Mail' to 'Intercept with MailCatcher' in the PHP-FPM component settings. See PHP-FPM Configuration for more info.