You must perform this procedure on e-mail clients that will use the POP3 service

1)Click Start, point to All Programs, and then click Outlook Express.
2)On the Tools menu, click Accounts.
3)Click Add, and then click Mail
4)Type Your name in Display name as you would like it to appear.
5)Type your email address people use to send email messages to you.
6)Type mail.domainname under incoming mail(POP3, IMAP OR HTTP) server as well as under Outgoing mail SMTP server.
For example if your domainname is example.com then type under incoming mail(POP3, IMAP OR HTTP) server, mail.example.com and under Outgoing mail SMTP server, mail.example.com.
7)Follow the instructions as above in the Internet Connection Wizard until you reach the Internet Mail Logon page
8)In Account name, type your POP3 service user name, followed by the domain, for example, someone@example.com.
In Password, type the password associated with your POP3 service e-mail account.
9)Click on properties button.
10)Enable: My servers requires authentication in servers tab
11)Enable: Leave a copy of message on the server in advanced tab.

While sending emails from your email address and you get the error Unrouteable address with RCPT reject. That means the problem at receivers end.

Check the following things.

1. Check the domains entry is available in userdoamins.

2. Check the domains entry is is available in localdomain

3. check the domain must having catchall address set if not then set
vi /etc/valiases/domain.com

*: username

4. also make sure that the mail folder having ownership user:mail

5 restart exim services once.

6. Now your problem will get resolved.

MX Record is an entry in a domain name database that identifies the mail server that is responsible for handling e-mail for that domain name.You can have website on one server and MX record on some another server and in that case you will have to ulter your MX entry in DNS zone for that perticular domain.

You can have multiple MX records for a same domain and it will check for MX record set with priority.
lowest number will receive mail first that is lower number will have highest priority.

1)You will have to login as a root in WHM

2)Go to the option called DNS Functions >> Edit DNS Zone

3)Select the domain from list and edit that domain
You will see the entries like
XYZ.com. 14400 MX 0 XYZ.com.
mail 14400 CNAME XYZ.com.

4)Suppose you want to change MX to point to the ip 176.199.1.90 then you will have to do the changes in MX record as
XYZ.com. 14400 MX 0 mail.XYZ.com.
mail.XYZ.com. 14400 A 176.199.1.90

5)Save the chnages and remove the entry for XYZ.com from /etc/localdomains.

6)Edit the /etc/remotedomains file - this tells Exim to not handle mail for this domain.
vi /etc/remotedomains
Add the domain name to this file.
Save the changes.

7)You can check the changes done in MX record for the domain at — http://www.dnsstuff.com/