`message:shrink_data()`

Since: Version 2025.03.19-1d3f1f67

The functionality described in this section requires version 2025.03.19-1d3f1f67 of KumoMTA, or a more recent version.

This method will ensure that the message contents are journalled to the spool, and then release any in-memory body data, keeping the metadata in-memory.