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

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

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.

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.