Bug#315180:

[sam]

Package: exim4-config
Version: 4.50-8

In acl/30_exim4-config_check_rcpt

# Accept if the address is in a domain for which we are relaying, but  
again,
   # only if the recipient can be verified.
   #
   # If you want to use the more conservative "unknown user" error
   # message in case of a non-existing local part, you might want to
   # set CHECK_RCPT_GIVE_UNKNOWN_USER. However, this might reveal
   # local information, which is the cause for it not being enabled by
   # default.
   accept
     domains = +relay_to_domains
     endpass
     .ifdef CHECK_RCPT_GIVE_UNKNOWN_USER
     message = ${if eq{$acl_verify_message}{Unrouteable address} 
{unknown user}{$a
cl_verify_message}}
     .else
     message = unrouteable address
     .endif
     verify = recipient


shouldn't the last line be change to:

     verify = recipient/callout=defer_ok

In other words, merely doing a recipient verify on a domain for which  
you relay doesn't really return any useful information. (Unless I'm  
missing something).  A callout is what I'd think you want.  I could  
go either way on the defer_ok option, but my preference is not to  
generate 4xx messages  when acting as a relay.