[exim] Strange behaviour using non default port in remote sm…

Hi, I'm using FreeBSD 8.3 amd64 and exim-4.76 with the following
compilation options:

      CDB=on: Enable CDB-style lookups
      DAEMON=on: Install scripts to run as a daemon
      DISABLE_D_OPT=on: Disable macros overrides using option -D
      DKIM=on: Enable support for DKIM
      DNSDB=on: Enable DNS-style lookups
      EMBEDDED_PERL=on: Enable embedded Perl interpreter
      ICONV=on: Enable header charset conversion
      LMTP=on: RFC2033 SMTP over command pipe transport
      LSEARCH=on: Enable wildcarded-file lookups
      MAILDIR=on: Enable Maildir mailbox format
      OLD_DEMIME=on: Enable old, deprecated demime ACL
      SUID=on: Install the exim binary suid root

I have the following router:
   internal:
     driver = manualroute
     domains = +relay_to_domains
     transport = remote_smtp
     hosts_randomize = true
     route_data = 
${lookup{$domain}lsearch{/usr/local/etc/exim/remote-servers}}
     no_more

and the related transport:

   remote_smtp:
     driver = smtp

In /usr/local/etc/exim/remote-servers I have:

domain1.tld: internal_smtp1::1026:internal_smtp2::1026
domain2.tld: internal_smtp1::1027:internal_smtp2::1027
domain3.tld: internal_smtp1::1028:internal_smtp2::1028


Everything just is running fine but sometimes (3/4 times every day) a
mail to any.user@??? is routed to internal_smtp1:1028 or
internal_smtp2:1028.

Now I'm testing route_list instead of route_data (to avoid the lookup
operation).

Is there anything I could check?

Thanks,
d.