**TL;DR:** Emails can be at most 40MB including attachments after Base64 encoding, and the batch endpoint does not support attachments at all. An oversized or batch-sent attachment fails instead of sending. Confirm the cause: check the total email size. Check the endpoint: if you are using the batch send endpoint, attachments are not supported there at all.

## The error

```text
a send with attachments fails, or attachments silently do not work via the batch endpoint.
```

## The fix

1. Confirm the cause: check the total email size. The limit is 40MB for the whole email including attachments after Base64 encoding, and Base64 inflates files by about a third, so a 30MB file is already over the limit once encoded.

2. Check the endpoint: if you are using the batch send endpoint, attachments are not supported there at all. Move those sends to individual email API calls.

3. Fix oversized mail: compress the file, link to a hosted download instead of attaching, or split it across multiple emails, keeping each email under 40MB after encoding.

4. The same limits apply to inline embedded images: they count as attachments for sizing.

5. Verify: re-send and confirm a 200 with an email id, and confirm the recipient receives the attachment intact.

## When to use this

- You are seeing this exact error message; match the block above, not just part of it.
- The failing call matches the scenario in the title: Resend attachment rejected.
- You want the fastest verified fix before digging through logs.

## When not to use this

- Your error text differs from the block above; close cousins often have different causes.
- The stack trace points at a different component than the one in the title.
- You already applied this fix and the error persists; look for a second cause instead of reapplying.

## Compatibility

- Not pinned to a specific version; follows current Resend behavior.