In LotusScript, use the CopyToDatabase method of the NotesDocument class to copy a document into mail.box (on the server) without any Paste agents or server tasks overwriting any fields. The document must have the SendTo, Recipients, From, Subject, and Body field set properly. The Form field must be set to "Memo".
You also use the CreateDocument method of the NotesDatabase class to create a document directly in mail.box. The message will be routed as soon as you call the Save method.
This can only be done on the server. If it is done via the client, the From field will be overwritten during the replication process.
Alternatives are also described in Lotus' Agent FAQ by JulieK.