kumomta/docs/reference/message/import_x_headers.md

message:import_x_headers([NAMES])

When called with no parameters, iterates the headers of the message, and for each header with an "X-" prefix, the import the header into the message metadata.

When called with a list of header names, only those headers, if present in the message, will be imported to the message metadata.

When importing an X- header, the header name is normalized to lowercase and - is transformed to underscore.

For example, with a message content of:

X-Campaign-ID: 12345
X-Mailer: foobar
Subject: the subject

The body

calling:

message:import_x_headers()
print(message:get_meta 'x_campaign_id') -- prints 12345
print(message:get_meta 'x_mailer') -- prints foobar

but calling:

message:import_x_headers { 'x-campaign-id' }
print(message:get_meta 'x_campaign_id') -- prints 12345
print(message:get_meta 'x_mailer') -- prints nothing