[exim] MX lookup for local domains

Hi all!

First I want to apologize if this question has already been answered
here. I've searched the archives but did not found the answer...

I have a server for shared hosting in which the costumers have the
ability to add or change domains via the web hosting control panel.
Being so, the list of local domains is not always reliable since it may
contain a domain that is not hosted in the server. However, the DNS
server is external and is reliable, so I thing that doing an MX lookup
should resolve the problem.

In the Exim configuration file, at my RCPT acl, I added the line
"domains = @mx_primary" and now have this:

   accept  domains = +local_domains
           domains = @mx_primary   
           endpass
           message = "Unknown User"
           verify = recipient

At my lookuphost router, I commented the line "domains = !
+local_domains" and added the line "self = pass" and, so I now have
this:

   lookuphost:
      driver = dnslookup
      #domains = ! +local_domains
      ignore_target_hosts = 127.0.0.0/8
      transport = remote_smtp
      self = pass
      no_more

As I don't have much experience with Exim configuration I would really
appreciate your thoughts on these modifications. Do these modifications
seem OK to you or may they break something?

TIA!

Regards,

--
Miguel Saturnino

Teia Digital (R)     Alojamento de Sites e Soluções Internet
   Web Site:         http://www.teiadigital.pt
   E-mail:           info@???