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.
• "authz_id" - the authorization id if the message was received via authenticated SMTP
• "authn_id" - the authentication id if the message was received via authenticated SMTP
• "reception_protocol" - "ESMTP" or "HTTP"
• "received_via" - the address:port of the local machine which received the message. Currently only set for SMTP receptions.
• "received_from" - the address:port of the peer address from which we received the message
• "routing_domain" - {{since('2023.08.22-4d895015', inline=True)}}. Overrides the domain of the recipient domain for routing purposes.