How to test mail app feature that enables the user to open few mails at once

I have to test this new feature that is similar to what exist on the native mail app of iOS.

The features enables the user to return from a new mail to the inbox and open a mail from the mail list, copy something and return to the new mail and paste it.

The feature also enables the user to write few mails in parallel, to switch between them and to copy-paste between them.

The feature also enables the user to choose between the mails that are open and to delete them(while writing).

I wrote already some tests, but they seem to be simplistic and I am looking for more complex tests.