This is the mail.yml file from a recent site (Trellis v0.9.4):
mail_smtp_server: smtp.sendgrid.com:587
mail_admin: tech@thebluecube.com
mail_hostname: thebluecube.com
mail_user: bcwordpress
mail_password: "{{ vault_mail_password }}" # Define this variable in group_vars/all/vault.yml
For mail user and password I created new “credentials” (and gave it Mail access) just for this specific site.