Mailgun settings for multiple sites on a single machine

I have this in my mail.yml:

# Documentation:
mail_password: "{{ vault_mail_password }}" # Define this variable in group_vars/all/vault.yml

How can it be setup for multiple sites? My problem is with mail_user and mail_admin variables, since they require a domain part in them, and I have just a single Trellis folder, so a single mail.yml file.
Is it even possible with multiple sites on a single machine?

Trellis only supports defining mail settings for the entire server

You’ll want to use a WP plugin for handling SMTP if you want different settings per site

But there seems to be a line in ssmtp config which says:
FromLineOverride={{ ssmtp_from_override }}

Would this allow external programs, like CF7 in WP to set the FROM address accordingly? So each site would have | | … in mails sent with contact forms?
I would verify all domains on the machine with Mailgun DNS entries, so there shouldn’t be a problem with that.