• 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 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.

• 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