testmxlookup — Look up mail relays for a domain
testmxlookup  [@ip-address] [--dnssec] [--udpsize n] {domain}
testmxlookup lists the names and IP addresses of mail
relays that receive mail for the domain.
This is useful in diagnosing mail delivery problems.
testmxlookup sends a DNS MX query for the specified domain, followed by A/AAAA queries, if needed. testmxlookup lists the hostname and the IP address of every mail relay, and its MX priority.
The error message "Hard error" indicates that the domain does not exist, or does not have any mail relays. The error message "Soft error" indicates a temporary error condition (usually a network failure of some sorts, or the local DNS server is down).
@ip-address
	      Specify the DNS server's IP address, where to send the DNS
	      query to, overriding the default DNS server addresses read from
	      /etc/resolv.conf.
	    
“ip-address” must be a literal, numeric, IP address.
--dnssec
	      Enable the DNSSEC extension. If the DNS
	      server has DNSSEC enabled, and the
	      specified domain's DNS records are signed, the list of
	      IP addresses is suffixed by “(DNSSEC)”, indicating
	      a signed response.
	    
This is a diagnostic option. Older DNS servers may respond with an error, to a DNSSEC query.
--udpsize n
	      Specify that n is the largest
	      UDP packet size that the DNS server may
	      send. This option is only valid together with
	      “--dnssec”.
	      If “--dnssec” always returns an error, try
	      “--udpsize 512” (the default setting is 1280
	      bytes, which is adequate for Ethernet, but other kinds of
	      networks may impose lower limits).