Pinging a domain name is not an especially good indicator - there are a number of scenarios where a domain name will not respond to pings, but can accept mail.

• Firewall set up to allow incoming traffic on TCP 25 (the port for SMTP) but block ICMP traffic (e.g. pings). Many firewalls block ICMP traffic by default.
• Yo-yo server - because of the way sending mail servers keep retrying mail, the receiving end only needs to be alive 50% of the time to offer a fairly reliable mail service. So even if the server is down (and not responding to pings), you could still send e-mail to it.
• A DNS MX record exists for a host but no A/CNAME record. Given the following DNS configuration:

  example.org.      MX 10    mail.example.org.
  mail.example.org. A        10.10.10.1.
  www.example.org.  A        10.10.10.2.
  

  You will end up with a situation where mail.example.org and www.example.org will accept pings, but example.org will not accept pings (unknown host), yet addresses like joe@example.org can still be received.

So don't ping the domain. Use a Perl module like Net::DNS::Resolver to look up an MX record for the host; if there is no MX record, then look for an A/AAAA/CNAME record; if you've still not found a matching DNS entry, then the domain name probably can't receive mail.

use Net::DNS::Resolver;

my $domain = 'example.com';
my $dns    = Net::DNS::Resolver->new;
my $valid  = $dns->query($domain, 'MX')
             || gethostbyname($domain);
printf(
   "You could %s send mail to %s.\n",
   ($valid ? "maybe" : "not"),
   $domain,
   );
[download]