add a domain to exim mail server
To receive a mails for a domain, you need to add the domain to exim mail server.
This is done by editing exim.conf file.
To accepts mails for webcash.in domain, i have pointed MX record of the domain to the server that runs exim mail server.
Then edited /etc/exim.conf file, added the domain to end of local_domains list.
domainlist local_domains = @ : localhost : localhost.localdomain : webcash.in
Restarted the exim.
Now all incoming mails will be delivered to local system (linux users created with adduser command) users.