Bulk Email Sending Rate Limited in Windsurf App
Your Windsurf-generated app tries to send emails in bulk — newsletters, notifications, invitations — and hits rate limits from the email provider. Some emails send while others silently fail, bounce, or get queued indefinitely. Your SMTP provider may temporarily block your account.
Cascade often generates email sending code that loops through recipients and fires off emails as fast as possible, without any throttling, queuing, or error handling for rate limits. This works fine when sending a few emails in development but fails catastrophically when sending to hundreds or thousands of recipients.
The consequences are serious: users don't receive important notifications, your email domain reputation drops, and your account may be suspended by the email provider.
Error Messages You Might See
Common Causes
- No rate limiting on send loop — Cascade generated a forEach loop that sends all emails simultaneously without any delay
- SMTP provider limits exceeded — Free tiers typically allow 100-300 emails/day. Even paid plans have per-second and per-minute limits
- No queue system — Emails are sent synchronously in the request handler instead of being queued for background processing
- Individual sends instead of batch API — Each recipient triggers a separate SMTP connection instead of using batch sending APIs
- No retry with backoff — When a send fails due to rate limiting, the code doesn't retry with exponential backoff
- Domain reputation degraded — Sending too many emails too fast triggers spam filters, causing future emails to be blocked even under the rate limit
How to Fix It
- Implement an email queue — Use BullMQ, Agenda, or a similar job queue to process emails in the background with configurable concurrency
- Add send rate throttling — Limit sends to your provider's rate: e.g., SendGrid allows 600/minute on free tier. Add delays between batches
- Use batch sending APIs — Switch from individual SMTP sends to the provider's batch API (SendGrid v3, Postmark batch, SES bulk) which handles rate limiting internally
- Implement exponential backoff — When you get a 429 rate limit response, wait and retry with increasing delays: 1s, 2s, 4s, 8s
- Segment large sends — Break a 10,000-recipient campaign into batches of 100, with pauses between batches
- Monitor delivery metrics — Track bounce rates, open rates, and spam complaints. If bounces exceed 5%, stop sending and clean your list
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Get HelpFrequently Asked Questions
How many emails can I send per day on free tiers?
Typical free tier limits: SendGrid 100/day, Resend 100/day, Mailgun 100/day (first 3 months: 5000/month), Amazon SES 200/day. For bulk sending, you'll need a paid plan and should ramp up volume gradually over days.
Should I use SMTP or an API for bulk email?
Always use the provider's HTTP API for bulk sending. APIs handle connection pooling, rate limiting, and batching internally. SMTP requires managing individual connections, is slower, and is more likely to hit rate limits.