kumomta/docs/reference/headermap/append.md

append

headers:append(NAME, VALUE)

{{since('2025.12.02-67ee9e96')}}

Constructs a new header with NAME and VALUE and appends it to the header map.

If the header value is ascii then it will be soft wrapped at whitespace around 75 columns, and hard-wrapped regardless of whitespace at 900 columns.

If the header value is non-ascii then it will be quoted printable encoded using RFC 2047 header encoding.

headers:append('X-Something', 'Some value')