Reject Invalid Email

This will be helpful in rejectig the invalid email id's in the server.

1. Take a backup of the /etc/exim.conf

2. vi /etc/exim.conf

3. Look for something like the following.
accept domains = +local_domains
accept domains = +relay_domains
accept hosts = +relay_hosts
accept condition = ${perl{checkrelayhost}{$sender_host_address}}

Between
accept domains = +local_domains

and
accept domains = +relay_domains

add the following:
endpass
message = unknown user
verify = recipient

The final section will look like this:
accept domains = +local_domains
endpass
message = unknown user
verify = recipient
accept domains = +relay_domains
accept hosts = +relay_hosts
accept condition = ${perl{checkrelayhost}{$sender_host_address}}

Save the modified file and restart the exim service.