I’ve recently changed my network connection at home to a provider which assigns dynamic addresses. Exim always provided a broken HELO/EHLO name to my smarthost since then because my externally visible hostname changes each time I connect. I’m now using Exim’s Perl interface to lookup the assigned hostname when connecting my smarthost:

• /etc/exim4/exim.pl:
  Don’t forget to change ppp0 to the interface you want to handle!

  #! /usr/bin/perl
  
  # Requires libio-interface-perl
  
  use strict;
  use IO::Socket;
  use IO::Interface;
  
  sub get_remote_helo_data()
  {
      my $s = IO::Socket::INET->new(Proto => 'udp');
      my $addr = inet_aton($s->if_addr('ppp0'));
      my $hostname = gethostbyaddr($addr, AF_INET);
  
      $hostname = '' if (!$hostname);
  
      return $hostname;
  }
  

• /etc/exim4/conf.d/main/50_exim4-localconfig_perl:

  #main/50_exim4-localconfig_perl
  perl_at_start = true
  perl_startup = do '/etc/exim4/exim.pl'
  

• Add the following code to the appropriate transport, e.g. remote_smtp_smarthost:

  helo_data = \
    ${if >{${strlen:${perl{get_remote_helo_data}}}}{0} \
                   {${perl{get_remote_helo_data}}} \
                   {$primary_hostname}}