The third step is to define which email address you'd like your subscribers to see as the sender, and when they hit reply, which address to be automatically populated.

For example: you might be sending from a marketing address, but if the user responds, you'd like the response to go to your support address.

As per above you can add, deactivate, and edit "From" and "Reply to" addresses for the account.

In terms of confirming which addresses are used when sending, you can do this when creating a new campaign.