[exim] Persistent storage per-message

Hi,

When routing an address in a router, is there a way to store a flag in a
variable/header such that when the message is finally delivered by a
transport to one of the other recipients, the transport's behaviour can
depend on that flag? I'm trying to get the presence of an alias-resolved
recipient to affect the behaviour of delivery to other recipients of a
message.

thanks in advance,

Matthew.