Using Vapor with different mail provider to SES (Mailgun/postmark etc.)

Is anyone aware how/if this is possible? I have set mail: false in yapor.yml which is supposed to disable SES and I have setup environment variables for my mailgun credentials.

When I try to send an email, I get an SES exception (this is because our domain is not verified). However, I would not expect at this point we even be trying to use SES as I have disabled it and specified mailgun.