Command Center

For end-users:

* Check with your registrar and make sure that the domain name hasn't expired.
* Make sure that your email inbox and/or domain didn't exceed its quota.
* Use your full email address Code by which the Internet identifies you. The format is username@hostname, where username is your username, login name, or account number, and hostname is the name of the computer or Internet provider you use. The hostname may be a few words strung together with periods. as username (i.e., username@domain.com).
* Switch to a different port such as: 26 or 1000 for SMTP Outgoing Server .
* Login to your control panel and remove any mail filter you have.

If you can't send and/or receive using MS Outlook/Outlook Express do the following:

1. Make sure that "My Server Requires Authentication" is checked in your Outlook/Outlook Express.
2. Make sure your Anti-Virus and/or Windows firewall are not blocking you from sending/receiving email
3. Go to your DOS prompt and issue the following command: ipconfig /flushdns
4. Run Windows Disk Defragmenter and wait until it is completed.
5. Power off, wait for a few seconds, and then power on your PC.
6. Turn off any firewall you might have on your own PC, including Norton and McAfee.

For System Administrators:

* SSH to your server and check Exim main log file for errors that might help in solving the problem:
For the cPanel/WHM
tail -f /var/log/exim_mainlog
For DirectAdmin
tail -f /var/log/exim/mainlog

* Go to either www.dnsreport.com or www.intodns.com and get a DNS report about the domain with the problem. Look carefully for any errors in the report and correct them.
* Make sure your server resolves, and the entries in these two files are correct: /stc/resolv.conf and /etc/hosts
* Make sure the domain name in question is not due for the annual domain name registration
* Ask your host to set rDNS PTR Records on your main IP address
* Make sure client's DNS zone and nameservers are correct
* Make sure that the email filter and/or SpamAssassin are configured and working properly
* Remove any mail filter created by the client.
* Verify that the email and/or the domain exist where they should exist
For a cPanel/WHM powered server
/etc/localdomains
* Make sure /etc/exim.pl exists

For the cPanel/WHM v11.x
Login to the WHM >> Main >> Service Configuration >> Exim Configuration Editor
Scroll down the page and click on the button "Reset All Configs to Default" then SSH to the server and run these commands at the prompt:

/scripts/buildeximconf
/scripts/mailperm
/scripts/eximup --force

Reference: http://servertune.com/kbase/entry/34/