Whenever a local transport is run, Exim forks a subprocess for it. The values used for the uid, gid, and the directories may come from several different places. In many cases the director that handles the address associates settings with that address. However, values may also be given in the transport's own configuration, and these override anything that comes with the address. The sections below contain a summary of the possible sources of the values, and how they interact with each other.
All local transports have the options group and user .
If group is set, it overrides any group that may be set in the address, even if user is not set. This makes it possible, for example, to run local mail delivery under the uid of the recipient, but in a special group. If user is set for a transport, its value overrides what is set in the address. If user is non-numeric and group is not set, the gid associated with the user is used. If user is numeric, group must be set.