I am unable to access the webmail using my domain name ?

Solution :

[1] Check whether the name servers are pointed to our server properly.
Note : If you have changed the nameservers recently, it will take 24 to 48 hours to reflect which depends on the ISP.
[2] Check whether you are giving the domain name without any spelling mistakes
[3] Till the name server reflection you can use http://webmail.servername.com to access the mails.