A tip for those affected. Set up an account on fastmail or protonmail and set them as secondary MX in DNS. When your main site stops getting attacked, then disable the MX record. If doing this, remember to update your SPF DNS records as well. Both of these providers give instructions, but keep the MX number higher than your main provider that is being DDoS'd. You could temporarily set up a catch-all account on the secondary rather than setting up all your aliases.