How to modify the host name that a user read when connect from telnet to my server ? (SMTP and POP3 and other services )

Example for SMTP:

$ telnet x.x.x.x 25

Trying x.x.x.x
Connected to x.x.x.x
Escape character is '^]'
220 mailtest.example.com ESMTP Postfix


I want change "mailtest.example.com" when other name.


Thanks

ByE
MaX