A mail server publishes its 'name' in several ways:
- when sending a HELO or EHLO command to a remote mail server, it will say something like EHLO mail.server.name
- when an SMTP client connects to it, it will say something like 220 mail.server.name SMTP server ready
The mail server name should, ideally, be the DNS name of the mail server because some other mail servers will check this.
To change the server name in VPOP3, go to Settings -> Misc Settings -> General and set the VPOP3 Host Name setting.
In VPOP3 version 6.5 or later, you can override this on an SMTP Service basis by going to Services -> SMTP Server -> Advanced and set the Host name there. (If this is left blank, then VPOP3 will use the host name specified in the Misc Settings.