Quick answer
Resend documents post-encoding message size as 40 MB. The limit applies after base64 encoding, so source binary files need headroom.
Verified Aug 22, 2026Official source
Current limits
| Constraint | Current value | Scope | Verified source |
|---|---|---|---|
| post-encoding message sizeThe limit applies after base64 encoding, so source binary files need headroom. | 40 MB | Single email | ResendAug 22, 2026 |
Why does this limit matter?
A binary attachment below 40 MB can exceed the final message limit after encoding and MIME overhead.
This value is scoped to Single email; a different plan, runtime, model, endpoint, region, or account can produce a different effective constraint.
What should you check?
- Estimate base64 expansion and total MIME message bytes before send.
- Confirm the exact plan, model, runtime, endpoint, region, and account that serve the failing workload.
- Record the observed value, response headers or configuration, timestamp, and source without logging secrets.
Important caveats
- Attachments are not supported in batch sending.
- Treat the official source and live account configuration as authoritative if they differ from this verified snapshot.
HyperObserve reports the documented platform constraint. Your application, SDK, gateway, provider, region, or account can impose a lower effective limit.
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