[exim] acl_c variables

Can I set an acl_c variable in a non-smtp ACL?

I want to set a variable for the sole purpose of passing it down to a
delivery agent (as an environment string). The information only makes
sense for a SMTP received message (and pertains to the connection, so it
should be acl_c and not acl_m), but I'd like to always set it to
_something_, for uniformity.

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