kumomta/docs/reference/headermap/set_references.md

set_references

headers:set_references(VALUE)

{{since('2025.10.06-5ec871ab')}}

Assign VALUE to the References header.

VALUE may be either a string or an array style table of Message-Id strings.

If you assign using a string, the string will be parsed and validated as being compatible with the References header before allowing the assigment to proceed.