kumomta/docs/reference/message/set_meta.md

message:set_meta(KEY, VALUE)

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

The set_meta method allows you to set a field of that object to a value that you specify.

You can assign any value that is serializable as a JSON:

-- set foo='bar', a string value
msg:set_meta('foo', 'bar')

-- set foo=123, a numeric value
msg:set_meta('foo', 123)

-- set foo=true, a boolean value
msg:set_meta('foo', true)

-- set foo={key="value"}, an object value
msg:set_meta('foo', { key = 'value' })

You can retrieve a metadata value via message:get_meta.

Pre-defined meta values

The following meta values have meaning to KumoMTA:

• "queue" - specify the name of the queue to which the message will be queued. Must be a string value.
• "tenant" - specify the name/identifier of the tenant, if any. Must be a string value.
• "campaign" - specify the name/identifier of the campaign. Must be a string value.