Oh, fascinating. Was it a Supabase thing? I haven't run into that before. If you have any more details, I'd love to hear. You could comment here or email team@slowsocial.us
supabase ceo here. Did you set up a custom SMTP? We have rate-limiting on the signup emails by default to protect against malicious users.
We also have some DDoS protection (via cloudflare), but it don't think this would have triggered it. Feel free to reach out to support if you need any assistance (https://app.supabase.io/support/new)