Raised by a customer.
Steps to reproduce:

I created a space and in Browse Space > Space Admin>Mail account > Add new Mail Account, I entered my work mail server name, account name, username and password. Now, all my emails are imported to this space and got deleted from my email server. BTW, I didn't do the import, I just click the test connection button and it imported all my emails.

A button that is named "Test Connection" should not download all emails.
Once in confluence it is very difficult to reverse