One common problem people have is an incorrectly setup mail system. Here is a list of rules that must be followed:
1) hostname must not match any domain that is being used on the system. Example, if you have a domain called domain.com and you want to recieve mail on user@domain.com, you must *not* set your hostname to domain.com. We recommend using server.domain.com instead. You must make sure that you add the A record for server.domain.com so that it resolves.
2) The hostname must be in the /etc/virtual/domains file.
3) The hostname must *not* be in the /etc/virtual/domainowners file.
4) The hostname must resolve. If not, add the required A records to the dns zone such that it does.
5) The directory /etc/virtual/hostname must exist.. (eg: /etc/virtual/server.domain.com). It must not contain any files.
6) Any domains that you want to use for email (eg: domain.com) must be in both the /etc/virtual/domains file and the /etc/virtual/domainowners file. The directory /etc/virtual/domain.com must exist and the files /etc/virtual/domain.com/passwd and /etc/virtual/domain.com/aliases exist.
7) File permissions for virtual pop inboxes should be (not applicable with Dovecot/Maildir):

/var/spool/virtual/domain.com 770 username:mail
/var/spool/virtual/domain.com/* 660 username:mail

If you’ve made any changes to you /etc/exim.conf file and require a fresh copy, you can retrieve one with this guide:
http://help.directadmin.com/item.php?id=51
Ensure your hostname does not contain any upper case letters.
9) Make sure that your main server IP has a reverse lookup on it.