Hi, you could setup a mail server with name "localhost" and ip address 127.0.0.1
In SH this should refuse all mail.
Then, ensure you "archive" all your other mail servers or set their priority to a much higher number than this one.
Once you're ready to re-enable outgoing mail, ensure you manually visit the outbound email queue as the "Super User" so you can delete any outgoing notifications from the queue or they will still get sent once you re-enable your real server.

Alternatively, you could use a service like:  Mailtrap - Email Testing Tool #1
This is a dummy SMTP server that catches all outgoing mail and lets you handle it separately, without it going out to the internet.