kumomta/docs/reference/message/set_due.md

message:set_due(due)

{{since('dev')}}

This method overrides the next delivery time for the message. The due parameter may be:

• nil - to indicate that delivery should be attempted as soon as possible.
• an ISO 8601 date and timestamp to specify the time of the next delivery attempt. msg:set_due("2024-03-08T17:51:42.481711Z")

Setting the due time is only valid in certain limited circumstances:

• Immediately at reception in either the smtp_server_message_received or http_message_generated events, although it is much simpler to use the message:set_scheduling method in almost all cases.
• During spooling in the spool_message_enumerated event, where it is anticipated that re-binding a message for immediate delivery would be the most likely use-case.

!!! warning Using this method in any other way can result in non-deterministic, undefined and unsupported behavior.