Exim will deliver to normal aliases, and aliases that are pipes or files, but it objects to aliases that involve :include: items, complaining that it can't change gid or uid. Why is this?

See `../Q0066`_ for a general answer. The problem happens during verification of an incoming SMTP message, not during delivery itself. In this particular case, you must have set up your aliasing router with a user setting. This causes Exim to change uid/gid when reading :include: files. If you do not need the detailed verification provided by the router, the easy solution is to set no_verify so that the router isn't used during verification. Otherwise, if you set user on the router in order to provide a user for delivery to pipes or files, one solution is to put the user setting on the transports instead of on the router. You may need to create some special transports just for this router. The alternative is to supply two different routers, one with user and no_verify, and the with verify_only but no user setting.