Solution:

• Open ESM drill down to
  
• Administrative groups
  
• Servers
  
• Your server
  
• Protocols
  
• SMTP
  
• Virtual Server, Properties
  

Deliver, and Advance, under Full qualified domain name, (VSEX1.smtp25.org) replace the entry with your public host record of your mail server FQDN (mail.smtp25.org), click Check DNS to make sure it is valid.

Stop and restart your Exchange Virtual server, now if you telnet into your server on port 25 you will get SMTP banner as mail.smtp25.org