Resend attachment rejected: 40MB total limit and no attachments on batch send

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

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.

Documented in Resend docs: emails can be no larger than 40MB including attachments after Base64 encoding; attachments are not supported on the batch endpoint; inline embedded images follow the same attachment limits. Evidence: resend.com/docs/dashboard/emails/attachments.