Thanks for jumping in. Composing is currently a paid feature, correct. You don't need a custom domain to activate it though, it also works with the in-house paced.email based email addresses.

Our underlying email provider is Mailgun and they handle the reputation. So far so good though, all seems to be working as intended a year and a half after launch.

Get in touch via the site if you want to chat more, happy to help if you have any questions etc.