We've seen this happen to a couple of people, looks like cPanel may have messed up an exim configuration file update. I think the solution is to reset the ACL configuration to the defaults. In WHM - Exim Configuration Editor, scroll to the bottom and click Reset ACL Config to defaults. If there were any modifications to the exim configuration you should add them back in. In this case I think it would only be the exim dictionary attack ACL, which you can get from the instructions here:
http://www.configserver.com/free/eximdeny.html
You only need to do the part referring to the WHM Exim Configuration Editor, not the bits that are done in shell as they should already be there, assuming that a MailScanner Service was done on this server.