PING when mail arrives

I block all pings to my mail/dns-server at the firewall. Now in my fw-logs I found that everytime I get a mail from a certain domain, I have two ping-entries in the logfiles. I found that it is the DNS-Server of the sender which is pinging me. 1. Why does the other DNS-Server ping me? (And send the mail after 2 failed attempts) 2. Should I allow ping to the mail/dns server? What implications would that have? Thanks Raffy