Select set up > sender addresses. By default, it is only possible to use the license localdomain. This means you can only choose what appears before the @ sign (e.g.: senderaddress@localdomain.com).
You can also use your own localdomain, which allows you to specify what appears after the @ sign. This feature must be requested from Webpower.
Note: As soon as you enter your own sender address instead of an email address with a local domain, this will affect deliverability. This is because mail from another domain with a custom sender adres, and this is not an address registered within Webpower, so you first need to request this at Webpower.