Checking for successful delivery

Mailman's deliver function will return {:ok, raw_delivered_message}, which contains this information. You can turn this raw string back into a %Mailman.Email{} struct using Mailman.Email.parse!:

{:ok, message} = MyApp.Mailer.deliver(email_with_attachments)
parsed_email = Mailman.Email.parse!(message)
delivered_date = parsed_email.delivery

At this point, if the deliver function added the Date header (meaning that it was accepted by the SMTP server) — then its value should show up in the delivery field.