$ host nasa.gov
nasa.gov mail is handled by 10 ndjsnpf01.ndc.nasa.gov.
nasa.gov mail is handled by 10 ndjsnpf02.ndc.nasa.gov.
nasa.gov mail is handled by 10 ndjsnpf03.ndc.nasa.gov.
nasa.gov mail is handled by 10 ndmsnpf01.ndc.nasa.gov.
nasa.gov mail is handled by 10 ndmsnpf02.ndc.nasa.gov.
nasa.gov mail is handled by 10 ndmsnpf03.ndc.nasa.gov.
So you can send mail to
email@example.com, but there is no machine
nasa.gov to connect to.
$ host www.nasa.gov
www.nasa.gov is an alias for www.nasa.gov.speedera.net.
www.nasa.gov.speedera.net is an alias for www.nasa.gov.edgesuite.net.
www.nasa.gov.edgesuite.net is an alias for a1718.x.akamai.net.
a1718.x.akamai.net has address 126.96.36.199
a1718.x.akamai.net has address 188.8.131.52
So when you ping
www.nasa.gov you are really pinging an Akamai machine via several indirections.
The NASA mail server is run by NASA directly. It doesn't reply to ping requests:
$ host ndjsnpf01.ndc.nasa.gov
ndjsnpf01.ndc.nasa.gov has address 184.108.40.206
$ whois 220.127.116.11
National Aeronautics and Space Administration NETBLK-NSI (NET-198-116-0-0-1) 18.104.22.168 - 22.214.171.124
$ ping ndjsnpf01.ndc.nasa.gov
PING ndjsnpf01.ndc.nasa.gov (126.96.36.199) 56(84) bytes of data.