kumomta/docs/reference/message/get_meta.md

message:get_meta(KEY)

Messages are associated with some metadata. You can think of this metadata as being equivalent to a JSON object.

The get_meta method allows you to retrieve a field of that object.

msg:set_meta('foo', 'bar')
print(msg:get_meta 'foo') -- prints 'bar'

See also msg:set_meta.