Right now, we are able to set up custom domains/subdomains for emails coming from email assist(example screenshots below).
However, these settings do not apply to when a user is sending emails from timeline. It sounds like this request may be on the roadmap for some new features but I wanted to make sure to get this out here on the community.
https://share.getcloudapp.com/4gu9Pe0k
Hello@alex_legay
Thanks for bringing this up. We will definitely add this to our roadmap and keep you posted about the updates.