Hi El Honrado Juan,
the “visible” sender of the emails (“From”) already uses the configured domain of your instance. Within the SMTP transaction the emails will be sent using the bounce.woltlab.cloud domain (“Return-Path”) so that we are able to handle bounce messages for you. This behavior is similar to common email service providers / newsletter providers.
Using an external SMTP server is not readily supported, but we could theoretically make this work upon request. Please either create a ticket or email email@example.com to work out all the details with my co-workers.