If you're using a custom domain and not receiving mail, first check that your domain is active. If your domain is not active, you won't be able to receive mail at any address using your custom domain. If your domain is not yet active, it means that we're waiting on a DNS change to be made. There are a few ways you can use a custom domain with Fastmail, which are detailed here.
If your domain is active, but mail is still bouncing, it may be related to another configuration in your domain.
To check your DNSSEC setup, check your domain set up using tools such as: