How can mails that are being routed through routers that do not set check_local_user be delivered under the uid of the recipient?

`../Q0601`_ contains background information on this. If you are using, say, an alias file to direct messages to specific mailboxes, you can use the user option on either the router or the transport to set the uid. What you put in the setting depends on how the required uid is to be found. It could be looked up in a file or computed somehow from the local part, for example.