You haven't binmode'd your socket, so I wonder how this works under Unix, and on Windows instead of sending \n you'll send \r\n to the socket.

For better debugging, consider printing the server answer in $_.

Have you considered simply using MIME::Lite instead? It works on Windows and systems with sendmail available and has been tested.

The value of $_ is 421 4.4.2 service timed out.
I tried binmode in many places on S and STDOUT, but maybe I didn't use it correctly.
I thought it maybe windows firewall so I turned that off, but it still does the same thing.
The better/faster way would be to use a module, so I will probably do that when I have more time to play with it.

Unrelated to your question but ($x, $x, $x, $x, $here) = gethostbyname($null); can be written (undef, undef, undef, undef, $here) = gethostbyname($null);.

yes and it can also be written like this.
($here) = (gethostbyname($null))[4];

Parentheses on the LHS not needed. (And around $null either.)

$here = (gethostbyname $null)[4];
[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]

I got the SMTP part to work with the code below.
Corion said to use binmode with \r\n, but I'm not sure if Socket is doing it. Because it seems to work with and without binmode now.
Update: FYI: The code below needs you to have a tab \t in the start of the message for it to work.

sub send_email {
 my ($self, %email_set) = @_;

 # Format input.
 $email_set{to}      =~ s/[ \t]+/, /g;
 $email_set{from}    =~ s/.*<([^\s]*?)>/$1/;
 $email_set{message} =~ s/^\./\.\./gm;
 $email_set{message} =~ s/\r\n/\n/g;
 $email_set{message} =~ s/\n/\r\n/g;
 $cfg{smtp_server} =~ s/\A\s+|\s+\z//g;

 # Send email via SMTP.
 if ($cfg{mail_type} == 1) {
 use Carp;
 use Socket;

  my $proto = getprotobyname('tcp') || 6;
  my $port  = getservbyname('SMTP', 'tcp') || 25;
  my $remoteserver = inet_aton($cfg{smtp_server})
    or croak 'Unable to resolve hostname : '.$cfg{smtp_server};

  croak 'Socket failure. '.$! if (! socket(S, AF_INET, SOCK_STREAM, $p
+roto));
  croak 'Bind failure. '.$! if (! bind(S, sockaddr_in(0, INADDR_ANY)))
+;
  croak 'Connection to '.$cfg{smtp_server}.' has failed. '.$!
   if (! connect(S, sockaddr_in($port, $remoteserver)));

  my $oldfh = select(S);
  $| = 1;
  select($oldfh);
  
  $_ = <S>;
  croak "Sending Email: data in Connect error - 220. $_ $!"
   if ($_ !~ /^220/);
   
  print S "HELO $cfg{smtp_server}\r\n";
  $_ = <S>;
  croak "Sending Email: data in Connect error - 250. $_ $!"
   if ($_ !~ /^250/);

  print S "MAIL FROM:<$email_set{from}>\r\n";
  $_ = <S>;
  croak "Sending Email: Sender address '$email_set{from}' not valid. $
+_ $!"
   if ($_ !~ /^250/);
   
  print S "RCPT TO: <$email_set{to}>\r\n";
  $_ = <S>;
  croak "Sending Email: Recipient address '$email_set{to}' not valid. 
+$_ $!"
   if ($_ !~ /^250/);
   
  print S "DATA\r\n";
  $_ = <S>;
  croak "Sending Email: Message send failed - 354. $_ $!"
   if ($_ !~ /^354/);
 }

 # Send email via sendmail.
 $ENV{PATH} = '';
 if ($cfg{mail_type} == 0) {
  open S, '| '.$cfg{mail_program}.' -t' or croak 'Mailprogram error. a
+t '.$!;
  }

  print S "To: $email_set{to}\r\n";
  print S "From: $email_set{from}\r\n";
  print S "Subject: $email_set{subject}\r\n";
  print S "$email_set{message}";
  print S "\r\n.\r\n";

 # Send email via SMTP.
 if ($cfg{mail_type} == 1) {
  $_ = <S>;
  croak "Sending Email: Message send failed - try again - 250. $_ $!"
   if ($_ !~ /^250/);
  print S "QUIT\r\n";
 }

 close(S);
 return 1;
}
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as readline teaches, $! is only significant if readline returned undef , if $_ is undef