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Message headers on Send response and new include_basic_headers option

Email

The Send Message endpoint now returns message headers on the response, and a new lightweight include_basic_headers option returns only the three RFC threading headers — Message-ID, In-Reply-To, and References. Together these eliminate the second GET /messages/{id} round-trip that customers previously needed to pull threading data after a send.

Added

Section titled “Added”

  • Headers on the Send Message response — Pass fields=include_headers or fields=include_basic_headers on POST /v3/grants/{grant_id}/messages/send and Nylas returns the message’s headers array on the response. This works for synchronous send only; the parameter has no effect when you set the send_at field. Supported across Google, Microsoft, EAS, EWS, and IMAP, with the same provider caveats that apply to include_headers on Get Message today.

  • fields=include_basic_headers option — Returns only Message-ID, In-Reply-To, and References in the headers array. The new value is available on:

    Use this option when you only need to track message identity and thread relationships. The response is significantly smaller than include_headers (full headers can be larger than the message body itself), and EWS returns the threading headers reliably with include_basic_headers — whereas include_headers on EWS only returns headers Nylas generates for MIME.

  • Using email headers and MIME data — Updated with the new option, a provider compatibility matrix, examples for both Get Message and Send Message flows, and a note that the Send response support is sync-only.