[Exim] verify recipients when being a mailhub

Hi all

after some diggin' in the net and old mailing lists, I solved it as
follows:

(...)
badusers:
driver = domainlist
domains = customer1.org:customer2.com
local_parts = lsearch;/etc/exim/badusers.${domain}
route_list = * localhost byname
self = local
verify_only

local_domains
  driver = domainlist
  transport = local_smtp
  route_list = customer1.org c1mail1:c1mail2 byname;\
               customer1.org c2mail1:c2mail2 byname

(...)
Now there exist files /etc/exim/badusers.customer1.org and
/etc/exim/badusers.customer2.com that contain entries like

foo:    :fail:  No such user here
bar:    :fail:  No such user here
...

-- Ben (tictactux at surfeu in ch)