this is only for me !

Tools to use:

First tool to look up the MX entry is nslookup ;)

type in the shell: nslookup -querytype=MX zentralweb.de e.g.
answeris:
Non-authoritative answer:
zentralweb.de   mail exchanger = 10 mail.zentralweb.de.

lets try some other options like A or CNAME

 nslookup -querytype=A zentralweb.de

Non-authoritative answer:
Name:   zentralweb.de
Address: 37.228.153.11

nice!

Second tool to check SMTP. For this i use telnet.
try in the shell: telnet zentralweb.de 25 be surprised!